Note maximum Markdown heading depth for docs

What does this MR do and why?

Add a note in the GitLab Documentation Style Guide that the preferred maximum heading depth is 3.

• Greater than that suggests the page is too complex and should be divided into smaller pages.

• The per-docs page navigation pane cannot display any sections at heading level greater than 5.

  For example: https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/administration/object_storage.md#L368
  Renders as: https://docs.gitlab.com/ee/administration/object_storage.html#azure-workhorse-settings-source-installs-only (not visible in right TOC)