Added "ISA Documentation" index page

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+The best way to understand a CPU is to take one apart. The Little Computer
+family was designed with exactly that in mind: every detail is intentional,
+every constraint has a reason. The three canonical architectures (LC-2, LC-3,
+and LC-3b) were built to be studied, not just used, and their internal structure
+reflects that philosophy at every level.
+
+At the core, all three share the same foundation. The address bus is 16 bits
+wide, giving a linear address space of 65,536 locations, no segmentation, no
+paging, no virtual memory. Each architecture uses eight general-purpose
+registers (R0-R7), a program counter (PC), and a 3-bit condition code register
+(CC) that tracks the sign of the last result written to a register: negative,
+zero, or positive. Every instruction is exactly 16 bits wide, with the top 4
+bits reserved for the opcode, a constraint that shapes the design of every
+instruction in the set.
+
+Each successive version refines this foundation rather than replacing it. LC-2
+and LC-3 pair the 16-bit address bus with a 16-bit data bus, meaning each memory
+location holds a full 16-bit word - 128 KiB of addressable memory in total.
+LC-3b narrows the data bus to 8 bits: memory locations hold a single byte,
+halving the addressable memory to 64 KiB and introducing byte-level memory
+operations absent in the other two. LC-3 and LC-3b also add privilege levels - a
+distinction between user mode and supervisor mode - which the original LC-2 does
+not have.
+
+What none of them have is equally telling: no flags register, no status
+register, no variable-length instructions, no complex addressing hierarchies.
+The simplicity is not a limitation, it is the point.
+
+Beyond the canonical three, two other architectures are documented here. LC-1
+predates LC-2 and represents the earliest iteration of the family: simpler, more
+constrained, and rarely discussed today. LC-3.2 sits at the other end: an
+unofficial extension that expands the address space to 32 bits and reduces the
+word size to 8 bits, bringing the family closer to the real-world architectures
+it was never meant to replicate.
+
+
+
+## How the Family Evolved
+
+{{% steps %}}
+
+### LC-1
+
+The starting point of the family. An accumulator-based architecture with a
+single register, a 13-bit address space, and just 8 instructions. Extremely
+minimal by design. Closer to a teaching toy than a real CPU.
+
+### LC-2
+
+LC-2 marks the transition from a single-accumulator model to a proper register
+file. The initial revision introduced 4 registers and N,Z condition codes. This
+is a first step away from the extreme minimalism of LC-1.
+
+The final revision settled on 8 registers (R0-R7) and three condition codes (N,
+Z, P), the blueprint that all subsequent canonical architectures would inherit.
+The result is an architecture that feels closer to early 8-bit CPUs like the
+Intel 8080: still simple, but no longer a toy.
+
+### LC-3
+
+The most widely known architecture in the family. The initial revision
+introduced a stack that grows toward zero, PC-relative offsets using sign
+extension instead of zero extension, and register-relative addressing with
+signed offsets.
+
+The revised version added privileged memory, available in two models: fixed and
+selectable. LC-3 has been adopted by numerous universities as the basis for
+introductory computer architecture courses.
+
+### LC-3b
+
+A variant of LC-3 with an 8-bit data bus instead of 16-bit, halving the
+addressable memory to 64 KiB but introducing byte-level memory operations.
+
+Working at the byte level makes LC-3b the closest of the canonical architectures
+to how real-world CPUs actually handle memory.
+
+### LC-3.2
+
+An unofficial extension, not created by Patt and Patel, that expands the address
+bus to 32 bits and narrows the data bus to 8 bits - the same combination used by
+early x86 processors. It is the only architecture in the family that bridges the
+gap between the LC teaching model and real-world CPU design, which is what makes
+it worth studying.
+
+{{% /steps %}}
+
+
+
+## At a Glance
+
+| ISA    | Data Bus Width | Address Bus Width | Memory  | Registers                          | Instruction Width         | Privilege Levels | Status      |
+|:------:|:--------------:|:-----------------:|:-------:|:----------------------------------:|:-------------------------:|:----------------:|:-----------:|
+| LC-1   | 16-bit         | **13-bit**        | 16 kiB  | **One 16-bit accumulator**         | 16-bit (**3-bit** opcode) | No               | **Legacy**  |
+| LC-2   | 16-bit         | 16-bit            | 128 kiB | Eight 16-bit registers (R0-R7)     | 16-bit (4-bit opcode)     | No               | Canonical   |
+| LC-3   | 16-bit         | 16-bit            | 128 kiB | Eight 16-bit registers (R0-R7)     | 16-bit (4-bit opcode)     | Yes (2)          | Canonical   |
+| LC-3b  | **8-bit**      | 16-bit            | 64 kiB  | Eight 16-bit registers (R0-R7)     | 16-bit (4-bit opcode)     | Yes (2)          | Canonical   |
+| LC-3.2 | **8-bit**      | **32-bit**        | 4 GiB   | **Eight 32-bit registers** (R0-R7) | 16-bit (4-bit opcode)     | Yes (2)          | **Variant** |
+
+
+
+## A Note on Sources
+
+All data in this section is sourced from primary references: the original books,
+course materials, and official publications by Patt and Patel. Every claim is
+cited. If you spot an error or know of a better source, contributions are
+[welcome via GitHub](https://github.com/little-emulator/docs/issues).
+
+
+
+## Explore the Architectures
+
+Each architecture in the family has its own dedicated reference page,
+documenting the instruction set, registers, memory model, and encoding in full
+detail.
+
+### Core Architectures
+
+<br>
+{{< hextra/feature-grid cols="2" >}}
+  {{< hextra/feature-card title="LC-2"  icon="chip" link="lc-2" >}}
+  {{< hextra/feature-card title="LC-3"  icon="chip" link="lc-3" >}}
+  {{< hextra/feature-card title="LC-3b" icon="chip" link="lc-3b" >}}
+{{< /hextra/feature-grid >}}
+
+### Beyond the Canon
+
+<br>
+{{< hextra/feature-grid cols="2" >}}
+  {{< hextra/feature-card title="LC-1"   icon="bookmark" link="lc-1"   subtitle="**Legacy**: The accumulator-based predecessor" >}}
+  {{< hextra/feature-card title="LC-3.2" icon="cube"     link="lc-3.2" subtitle="**Variant**: An unofficial 32-bit extension" >}}
+{{< /hextra/feature-grid >}}