Update and move release process (#15511)
Summary of the issue:
NV Access is intending to move most of the wiki into the repository, so changes are tracked better and easier to propose.
The release process wiki page is a prime candidate for migration.
The release process wiki is out of date, it also contains some information that is better suited to or duplicated in other documents.
Description of user facing changes
The release process wiki page is migrated to the repository.
The process is updated to include more information on the timing of the stages.
Information for testers, and developers are moved to the appropriate contributing docs.
