Docs: add custom descriptions to most relevant docs
What does this MR do?
- Adds a custom description to the most relevant docs.
- Reduces the number of errors on Google Console (for https://gitlab.com/gitlab-com/gitlab-docs/issues/194) before we resolve https://gitlab.com/gitlab-com/gitlab-docs/issues/200
- EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5603
Edited by Marcia Ramos