Add Docusaurus documentation guide with progressive disclosure

CLAUDE.md gets a brief summary with link; full details in DOCUMENTATION.md
covering structure, versioning, and writing conventions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: alexeyzimarev

CLAUDE.md

@@ -97,6 +97,10 @@ Use conditional compilation for TFM-specific APIs: `#if NET`, `#if NET8_0_OR_GRE
 - Guard TFM-specific tests with `#if NET8_0_OR_GREATER`
 - Test results: `test-results/<TFM>/<ProjectName>.trx`
 
+## Documentation
+
+Docusaurus 3 site in `docs/`. Dev server: `cd docs && pnpm start`. See [DOCUMENTATION.md](DOCUMENTATION.md) for full details on structure, versioning, and writing docs.
+
 ## Working with Issues
 
 - Avoid changing default behavior unless absolutely necessary

DOCUMENTATION.md

@@ -0,0 +1,43 @@
+# Documentation Guide
+
+The project documentation lives in `docs/` and uses **Docusaurus 3** with pnpm.
+
+## Commands
+
+```bash
+cd docs
+
+# Install dependencies
+pnpm install
+
+# Local dev server with hot reload
+pnpm start
+
+# Production build
+pnpm build
+```
+
+## Structure
+
+- `docs/docs/` — Current (next) version of docs, auto-generates sidebar from folder structure
+  - `docs/docs/usage/` — "Using RestSharp" guides (basics, client, request, execute, response, example)
+  - `docs/docs/advanced/` — Advanced topics (authenticators, configuration, serialization, interceptors, error handling)
+  - `docs/docs/intro.md` — Getting started page
+  - `docs/docs/changelog.md` — Release changelog
+- `docs/versioned_docs/` — Frozen snapshots for past versions (v110–v113)
+- `docs/versioned_sidebars/` — Sidebar configs for each frozen version
+- `docs/versions.json` — List of released doc versions
+- `docs/src/pages/` — Standalone pages (migration guide, support)
+- `docs/docusaurus.config.ts` — Site config (URL, navbar, footer, plugins, versioning)
+- `docs/sidebars.ts` — Sidebar config (autogenerated from directory structure)
+
+## Versioning
+
+Docs use Docusaurus versioned docs. `docs/docs/` is the "next" (unreleased) version. To cut a new version: `pnpm docusaurus docs:version vXXX`. This copies `docs/docs/` into `versioned_docs/version-vXXX/` and creates a matching sidebar file. Update `docusaurus.config.ts` to add the new version label.
+
+## Writing Docs
+
+- Markdown files with optional frontmatter (`sidebar_position`, `title`, `sidebar_label`)
+- Category ordering via `_category_.json` files in each directory
+- Code blocks use `csharp` language identifier for syntax highlighting
+- Edit current docs in `docs/docs/`; do not modify `versioned_docs/` unless backporting fixes