Start linting for unneeded dollar signs

What does this MR do?

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/64352#note_213359399, we can easily enable this lint test on our docs:

• https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md#md014

Related issues

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page.
• Change the header tag to h5, #####.

Moving docs to a new location?

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.