docs: add API guide

docs/index.md

@@ -17,6 +17,7 @@ If you are just trying to pick a board or download firmware, start with:
 - [Download and Flash Releases](./releases.md)
 - [Migration From xJARiD/MeshCore](./migration.md)
 - [Use the Repeater Web Panel](./web-panel.md)
+- [Use the Repeater Web API](./api.md)
 - [Custom CLI Commands](./custom-cli.md)
 
 ## Developer Notes