Add hint to GNUPGHOME environment variable (#20134)

* Add hint for GNUPGHOME environment variable

With #19732, the default location for the `.gnupg` folder has changed. To mitigate this breaking change, users can specify the home directory for gnupg via `$GNUPGHOME` environment variable to keep using their current location.

* Update docs/content/doc/advanced/signing.en-us.md

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

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>

docs/content/doc/advanced/signing.en-us.md

@@ -100,9 +100,9 @@ ideal UI and therefore subject to change.
 **Since 1.17**, Gitea runs git in its own home directory `[git].HOME_PATH` (default to `%(APP_DATA_PATH)/home`)
 and uses its own config `{[git].HOME_PATH}/.gitconfig`.
 If you have your own customized git config for Gitea, you should set these configs in system git config (aka `/etc/gitconfig`)
-or the Gitea internal git config `{[git].HOME_PATH}/.gitconfig`. 
-Related home files for git command (like `.gnupg`) should also be put in Gitea's git home directory `[git].HOME_PATH`. 
-
+or the Gitea internal git config `{[git].HOME_PATH}/.gitconfig`.
+Related home files for git command (like `.gnupg`) should also be put in Gitea's git home directory `[git].HOME_PATH`.
+If you like to keep the `.gnupg` directory outside of `{[git].HOME_PATH}/`, consider setting the `$GNUPGHOME` environment variable to your preferred location.
 
 ### `INITIAL_COMMIT`