Shift versioning and dch information out of howtoreleasepackages

The versioning information doesn't sit very well in the releasing
packages guide. Add this to the information on binary package
versioning to make a more holistic page on versioning.

Move the information on dch tags to the version_control guide where
it is mentioned.

The rest of this guide is very out of date. We have a number of other
guides on releasing packages, drop this one.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>