Add the 'ui.user' section to the cheat sheet (go-gitea#22249)

The `ui.user` ini section with its single setting is not yet mentioned
in the config cheat sheet.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

Author: chrullrich

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

@@ -239,6 +239,10 @@ The following configuration set `Content-Type: application/vnd.android.package-a
 - `NOTICE_PAGING_NUM`: **25**: Number of notices that are shown in one page.
 - `ORG_PAGING_NUM`: **50**: Number of organizations that are shown in one page.
 
+### UI - User (`ui.user`)
+
+- `REPO_PAGING_NUM`: **15**: Number of repos that are shown in one page.
+
 ### UI - Metadata (`ui.meta`)
 
 - `AUTHOR`: **Gitea - Git with a cup of tea**: Author meta tag of the homepage.