dep-14: add a section describing package version branches

Could you please add a section documenting how to name package version tracking branches?

The convention <vendor>/<version>/<suite> naming convention described in this MR is compatible with the one currently used by the kernel team and the Gnome Team for MozJS.

An alternative convention using a [<srcpkg>/][<version>/]<vendor>/<suite> scheme was discussed in this MR, but I've decided to revert to the initial proposal (above) for the following reasons:

• there is a significant risk of clash between e.g. 3.x/debian/trixie and a 3.x upstream branch, as both branches could not coexist in the same repository
• after discussing the options with several DDs it appeared that the <vendor>/<version>/<suite> was preferred to the other scheme.

The alternative scheme is currently used by the godot repository.

Other existing single-repo schemes that I would not recommend are:

• LLVM
• gcc
• boost
• nvidia-graphics-drivers

Multiple repositories (a single package version per repo) seem to be the most common current practice (including for Java packages). Some packages avoid the issue by generating the versioned metadata at build-time depending on the upstream version being built, others by never tracking more than a single version for a given suite.