docs: add a contributing guide and a documentation index (#3081)

A new contributor had nothing to start from: the README covers using
CodeGraph, and the build and test rules live in AGENTS.md, written for
agents. CONTRIBUTING.md now covers building from source, testing a change,
preparing a pull request and what happens to it. docs/README.md indexes the
guides and design notes, and the README links to both.

Every relative link and heading anchor was checked against main.

Reimplements PR 1931 by @mixxer. Also folds in the fork-and-upstream setup,
`npm run cli`, `npm run test:watch` and the version rule from PR 306 by
@marcelovani, and the commit format, the single-test command, the bug-report
list and the `/add-lang` pointer from PR 665 by @ymstar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/README.md

@@ -0,0 +1,26 @@
+# Documentation
+
+To install and use CodeGraph, start with the [project README](../README.md). To build it and send a change, see [Contributing](../CONTRIBUTING.md).
+
+## Guides
+
+| Topic | Where to start |
+|-------|----------------|
+| Architecture, build rules and platform validation | [Project guide](../AGENTS.md) |
+| Distribution and release packaging | [Bundling](../BUNDLING.md) |
+| What telemetry collects, and how to turn it off | [Telemetry](../TELEMETRY.md) |
+| Release history | [Changelog](../CHANGELOG.md) |
+| Entries held for the viewer's launch | [Viewer changelog](viewer-launch-changelog.md) |
+| What languages and frameworks cover, and what's left | [Framework coverage](design/framework-coverage.md) |
+| Validating a language or framework on real repositories | [Validation guide](AGENTS.md) and [coverage playbook](design/dynamic-dispatch-coverage-playbook.md) |
+| Checking a language by hand | [Language verification guide](SEARCH_QUALITY_LOOP.md) |
+| The browser viewer's design | [Viewer design spec](design/codegraph-ui-design-spec.md) |
+| The native extraction kernel | [Kernel design](design/native-extraction-kernel.md) |
+
+## Supporting material
+
+- [Design](design/): architecture notes and feature proposals. Some describe planned work rather than what has shipped.
+- [Benchmarks](benchmarks/): measurements and how they were taken. Read each against the revision and setup it records.
+- [Grammars](grammars/): notes and patches for the parser grammars CodeGraph builds itself.
+- [Plans](plans/): implementation plans.
+- [Discussion notes](from-chat/): supporting notes from design discussions.