Amendments for DEP-14

See commits for details (use the 'Commits' tab and 'Next' button to browse):

• 40548594 dep14: Stable branches can also have version prefix (debian/13-bookworm)
• 72efaa6a dep14: Be explicit that upstream branch names can be left as-is
• cc448b55 dep14: Explain import if upstream has both git tags and tarball releases

See for example the MariaDB, Galera and Entr repositories to see the branches and tags in action. In particular the MariaDB example is complex but manageable with DEP-14 with these small amendments.

Screenshot of gitk in Galera after latest release for even better visualisation: