Reorganize docs

## Problem

Our repository documentation has grown organically and now spans multiple locations and formats, which makes it harder to **discover**, **navigate**, and **use effectively**—especially when someone is new to the `azd` codebase or trying to build on top of it.

Recently, we invested in consolidating important knowledge into Markdown (e.g., translating Figma design docs into `.md`, adding an extensions style guide, and documenting guiding principles for building with/on the `azd` core). However, that information still isn’t presented in a way that’s easy to find or follow end-to-end.

This “tidy work” is intended to make the docs serve these audiences better:

- **Extension authors** building and publishing `azd` extensions, who need clear conventions, guiding principles, and “how we do things” references to make good decisions (including in ambiguous situations).
- **Contributors to this repo** (new and existing), who need a predictable onboarding path and clear contribution mechanics without hunting across directories.
- **Coding agents** (and the humans directing them) helping build extensions or contribute to core, who benefit from docs that are structured, scannable, and optimized for agent-assisted workflows (clear entry points, topic-based organization, and minimal ambiguity).
- **People and agents trying to understand the CLI via the repo**, who need a coherent mental model of `azd` (architecture, workflows, design decisions, and where to find authoritative guidance).

## What we want to do to optimize our docs

- [ ] Update the root-level contribution documentation (it currently links to a deeper `CONTRIBUTING.md`), so the repo has a clear, top-level entry point for contributors and agents.
- [ ] Keep the root instruction file under **200 lines**, so it stays readable, high-signal, and friendly for both humans and coding agents.
- [ ] Organize knowledge **by topic**, reducing duplication and “where is the real doc?” confusion.
- [ ] Make **guiding principles** clear, prominent, and easy to discover.
- [ ] Ensure guiding principles explicitly help with **future, less-defined decisions**, especially for **extension authors** (and agents assisting them).