Update release post handbook

Update the release post handbook with things learned from the latest release (9.4):

• Add performance improvements and upgrade barometer instructions, based on https://gitlab.com/gitlab-com/www-gitlab-com/issues/1558
• Add more steps to the “authorship” section (make things clearer and smoother for the following release posts leads)
• Copyedit, link to anchors, minor edits

Review Apps: https://update-release-post-handbok.about.gitlab.com/handbook/marketing/blog/release-posts/

Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/1558

@bikebilly @joshlambert @mydigitalself @victorwu @markpundsack, feedback is much appreciated! :)