Improve documentation for PAM and static deployment (#21866)

## Changes proposed in [referenced issue 21845][1]

- Expand PAM configuration description with working examples.
- Clarify `STATIC_URL_PREFIX` use (include "assets" and only works after
database has been initialized)
- Add note for HTTPS proxy support VIA Apache.

[1]: https://github.com/go-gitea/gitea/issues/21845

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -289,8 +289,13 @@ The following configuration set `Content-Type: application/vnd.android.package-a
    This includes CSS files, images, JS files and web fonts.
    Avatar images are dynamic resources and still served by Gitea.
    The option can be just a different path, as in `/static`, or another domain, as in `https://cdn.example.com`.
-   Requests are then made as `%(ROOT_URL)s/static/css/index.css` and `https://cdn.example.com/css/index.css` respective.
+   Requests are then made as `%(ROOT_URL)s/static/assets/css/index.css` or `https://cdn.example.com/assets/css/index.css` respectively.
    The static files are located in the `public/` directory of the Gitea source repository.
+   You can proxy the STATIC_URL_PREFIX requests to Gitea server to serve the static
+   assets, or copy the manually built Gitea assets from `$GITEA_BUILD/public` to
+   the assets location, eg: `/var/www/assets`, make sure `$STATIC_URL_PREFIX/assets/css/index.css`
+   points to `/var/www/assets/css/index.css`.
+
 - `HTTP_ADDR`: **0.0.0.0**: HTTP listen address.
   - If `PROTOCOL` is set to `fcgi`, Gitea will listen for FastCGI requests on TCP socket
      defined by `HTTP_ADDR` and `HTTP_PORT` configuration settings.