Docs: refactor doc general guidelines and style guidelines
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
- Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41281
- Move general guidelines related to the document structure/process from the doc style guide to the documentation guidelines doc
- Adjust the text for contextualization
- Add missing guidelines (from #41281 (closed))