docs: mention build.gradle in UCM methods

Merged username-removed-151489 requested to merge Bubu/fdroid-website:repomanifest into master

UCM in Tags and RepoManifest mode also searches build.gradle files for version information.

This can still be improved, i.e. the note about the location for the last build is probably not really relevant for build.gradle files.

Not sure what to do about text wrapping. 🤷

Edited by username-removed-151489