DOCS: add mention of swagger api reference (#8452)

It's(swagger api link) mentioned vaguely in the FAQ but IMHO missing from API usage page.

Author: borisovano

docs/content/doc/advanced/api-usage.en-us.md

@@ -68,6 +68,14 @@ curl -X POST "http://localhost:4000/api/v1/repos/test1/test1/issues" \
 As mentioned above, the token used is the same one you would use in
 the `token=` string in a GET request.
 
+## API Guide:
+
+API Reference guide is auto-generated by swagger and available on: 
+    `https://gitea.your.host/api/swagger`
+    or on 
+    [gitea demo instance](https://try.gitea.io/api/swagger)
+
+
 ## Listing your issued tokens via the API
 
 As mentioned in