Merge branch 'fix/contributing-mr-template' into 'master'

Replace links to the `Merge request description format` section with
up-to-date instructions on using the MR template built into the UI.

The `Merge request description format` section was removed in 2f7e28d1,
but links were left in the table of contents and in the MR description
instructions. Presumably, the section was removed because MR templates
are now provided by the GitLab UI.

See merge request !7228

CONTRIBUTING.md

@@ -19,7 +19,6 @@
         - [Technical debt](#technical-debt)
     - [Merge requests](#merge-requests)
         - [Merge request guidelines](#merge-request-guidelines)
-        - [Merge request description format](#merge-request-description-format)
         - [Contribution acceptance criteria](#contribution-acceptance-criteria)
     - [Changes for Stable Releases](#changes-for-stable-releases)
     - [Definition of done](#definition-of-done)
@@ -262,8 +261,11 @@ request is as follows:
 1. Submit a merge request (MR) to the `master` branch
 1. The MR title should describe the change you want to make
 1. The MR description should give a motive for your change and the method you
-   used to achieve it, see the [merge request description format]
-   (#merge-request-description-format)
+   used to achieve it.
+  1. If you are contributing code, fill in the template already provided in the
+     "Description" field.
+  1. If you are contributing documentation, choose `Documentation` from the
+     "Choose a template" menu and fill in the template.
 1. If the MR changes the UI it should include *Before* and *After* screenshots
 1. If the MR changes CSS classes please include the list of affected pages,
    `grep css-class ./app -R`