Improve reverse proxy documents and clarify the AppURL guessing behavior (#31003...

Improve reverse proxy documents and clarify the AppURL guessing behavior (#31003) (#31020)

Backport #31003 by wxiaoguang

Fix #31002

1. Mention Make sure `Host` and `X-Fowarded-Proto` headers are correctly passed to Gitea
2. Clarify the basic requirements and move the "general configuration" to the top
3. Add a comment for the "container registry"
4. Use 1.21 behavior if the reverse proxy is not correctly configured

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>