Handbook - content marketing - updates

Merged username-removed-236961 requested to merge handbook-marcia into master

Updates to Content Marketing - GitLab Blog

  1. All posts should have the author's name instead of "GitLab"
  2. Add a section "Acknoledgements" to the end of the post to thank contributor, when that's the case
  3. For guest writers: add a section "About the guest author" to the end of the post

cc/ @AshleyS / @amara