Update end-to-end style guide doc with new guideline

What does this MR do?

This MR addresses the missing documentation mentioned on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30962, which is part of https://gitlab.com/gitlab-org/quality/team-tasks/issues/168.

Also, it does some small adjustments in the existing document sections (heading adjustments.)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content