new: [docs] Add comprehensive API reference documentation

Complete docs.md covering all 140+ public methods with examples,
organized by MISP entity type. Updated README to link to it.

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

Author: iglocska

README.md

@@ -127,6 +127,10 @@ let params = SearchBuilder::new()
 let results = client.search(SearchController::Attributes, &params).await?;
 ```
 
+## Documentation
+
+For a complete API reference covering all 140+ methods with examples, see [`docs.md`](docs.md).
+
 ## Examples
 
 See the [`examples/`](examples/) directory: