Add 'authoring releases' section to `contributing.md`

[TysonRayJones]

This should include instructions on how to update the QuEST version in the source code.
This includes updating the numbers in at least:

• CMakeLists.txt (VERSION)
• version.h
• Doxyfile (PROJECT_NUMBER)

[TysonRayJones]

Of course manually updating these is terribly dangerous and should instead be done by a CI (run whenever devel merges into main?)