docs: consolidate docs/api/README.md into docs/API.md (#183)

* Initial plan

* docs: consolidate docs/api/README.md into docs/API.md

Co-authored-by: mfranzke <787658+mfranzke@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mfranzke <787658+mfranzke@users.noreply.github.com>

Author: Copilot

docs/api/README.md

@@ -57,6 +57,8 @@ css-if-polyfill/
 │   └── ...other examples...
 ├── docs/
 │   ├── API.md             # Complete API documentation
+│   ├── RELEASE_PROCESS.md
+│   ├── TEST_FIXTURES.md
 │   └── refactoring/       # Integration documentation
 └── README.md              # Updated with v2.0 features
 ```