Add API docs to main site

[matmair]

We currently do not ship API docs with the normal docs. As we provide a demo instance this is not a huge problem but having something well-integrated might be worthwhile. My suggestion would be https://rapidocweb.com once we switch over to swagger / OpenAPI.

[matmair]

Maybe also diffs: https://github.com/goauthentik/authentik/blob/main/Makefile#L71-L79
https://github.com/goauthentik/authentik/blob/c30aa908887fcde9a6fe5e3514151f000ccce063/Makefile#L71