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))

Preview:

• Docs guidelines
• Docs style guide

EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4750