Add a reference to upstream gbp docs

Author: msuchane

docs/maintainers/niche-package-maintenance/desktop/maintain-ubuntu-desktop-software.md

@@ -1,5 +1,5 @@
 ---
-relatedlinks: "[PackagingWithGit   -   Debian Wiki](https://wiki.debian.org/PackagingWithGit)"
+relatedlinks: "[PackagingWithGit   -   Debian Wiki](https://wiki.debian.org/PackagingWithGit), [Building   Debian   Packages   with   git-buildpackage](https://honk.sigxcpu.org/projects/git-buildpackage/manual-html/)"
 ---
 
 (maintain-ubuntu-desktop-software)=