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>

composer.json

@@ -36,7 +36,9 @@
     "laminas/laminas-httphandlerrunner": "^2.12",
     "nyholm/psr7": "^1.8",
     "nyholm/psr7-server": "^1.1",
+    "phar-io/composer-distributor": "^1.0.2",
     "php-cs-fixer/shim": "^3.91",
+    "phpdocumentor/shim": "^3",
     "phpstan/phpstan": "^2.1",
     "phpunit/phpunit": "^10.5",
     "psr/simple-cache": "^2.0 || ^3.0",
@@ -69,7 +71,8 @@
   },
   "config": {
     "allow-plugins": {
-      "php-http/discovery": false
+      "php-http/discovery": false,
+      "phpdocumentor/shim": true
     },
     "sort-packages": true
   }