Add never option to PUBLIC_URL_DETECTION configuration (#36785)

Follow up #34250

Docs: https://gitea.com/gitea/docs/pulls/353

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

custom/conf/app.example.ini

@@ -74,6 +74,7 @@ RUN_USER = ; git
 ;; especially when the Gitea instance needs to be accessed in a container network.
 ;; * legacy: detect the public URL from "Host" header if "X-Forwarded-Proto" header exists, otherwise use "ROOT_URL".
 ;; * auto: always use "Host" header, and also use "X-Forwarded-Proto" header if it exists. If no "Host" header, use "ROOT_URL".
+;; * never: always use "ROOT_URL", never detect from request headers.
 ;PUBLIC_URL_DETECTION = legacy
 ;;
 ;; For development purpose only. It makes Gitea handle sub-path ("/sub-path/owner/repo/...") directly when debugging without a reverse proxy.