Add API documentation generation with phpDocumentor (#184)

* Add API documentation generation with phpDocumentor

- Add phpDocumentor configuration and `docs` Makefile target
- Generate docs during CI to catch errors before releases
- Set up GitHub Pages with Jekyll for hosting generated docs
- Add GitHub Actions workflow to deploy docs to GitHub Pages on releases

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix phpDocumentor errors in type annotations

Change class-string union types to use phpDocumentor-compatible syntax
and reorder docblock tags so class descriptions precede `@author` tags.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix CI: pin phar-io/composer-distributor ^1.0.2

phar-io/composer-distributor 1.0.0 has a bug where it uses the wrong
package version when determining the phpDocumentor download URL,
causing CI to fail with 404 errors when using --prefer-lowest.
Version 1.0.2 fixes this issue.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Integrate markdown guides into phpDocumentor output

Configure phpDocumentor to generate both API documentation and user
guides in a single unified site. This replaces the previous Jekyll-based
approach where API docs were copied into the docs folder.

- Add `<guide>` configuration to `phpdoc.dist.xml`
- Create custom template with "Guides" navigation link
- Replace `docs/index.html` redirect with `docs/index.md` guide index
- Update workflow to publish `build/docs` directly without Jekyll
- Track `.phpdoc/template/` while still ignoring `.phpdoc/cache/`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fine-tuning for path, config, and layout

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

.gitignore

@@ -9,3 +9,7 @@ examples/**/sessions
 tests/Conformance/results
 tests/Conformance/sessions
 tests/Conformance/logs/*.log
+
+# phpDocumentor
+.phpdoc/build/
+.phpdoc/cache/