Update documents for Gitea behind reverse proxy. Fix some small bugs (some URLs are...

Update documents for Gitea behind reverse proxy. Fix some small bugs (some URLs are generated without sub-path) (#17320)

* Apache `ProxyPassReverse` only works for Location, Content-Location and URI headers on HTTP redirect responses, it causes more problems than it resolves. Now all URLs generated by Gitee have the correct prefix AppSubURL. We do not need to set `ProxyPassReverse`.

* fix url param

* use AppSubURL instead of AppURL in api/v1

Co-authored-by: techknowlogick <techknowlogick@gitea.io>