docs: update release process with homebrew instructions

Description

As part of #1011 (closed) this MR updates documentation explaining how to release a new glab version through homebrew.

Related Issues

• Resolves Update HomeBrew recipe to source from the GitLa... (#1011 - closed)
• Related to docs follow-up: fill in gaps in release process (#1150 - closed)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)