Document gem release tools

[AgaDufrat]

What

Updatehttps://docs.publishing.service.gov.uk/manual/publishing-a-ruby-gem.html dev docs page to include optional but recommended Enforce gem version bumps workflow. Also mention Gem version checker.

Update https://docs.publishing.service.gov.uk/manual/slack-integrations.html to include Gem version checker.

Why

We have 3 tools that should ease gem release process and keep our software up to date:

• Gem version checker
• Auto-release rubygem workflow
• Enforce gem version bumps workflow