Update footnote guidance

What does this MR do and why?

Following this discussion, there seems to be an agreement on moving away from HTML because:

• We cannot use relative links in HTML, which means our pipeline cannot check for possible broken links in footnotes.
• Switching from Markdown to HTML is error-prone (see Examples of incorrectly rendered footnotes).

We should aim for a pure Markdown solution (see gitlab-docs#1766 (comment 1772209687) and !144843 (comment 1794563027)). The reason HTML was introduced is that our footnotes looked like the start of an ordered list. A simple and boring solution is to add **Footnotes:** at the beginning so the footnotes wouldn't be mistaken for a list.

I've also taken Marcel's feedback into account by making clear that we should use footnotes only when we cannot include the information in the table itself.

Examples of incorrectly rendered footnotes

• https://docs.gitlab.com/ee/api/groups.html#list-a-groups-projects

  Footnotes:
  1. Order by similarity: Orders the results by a similarity score calculated from the provided `search` URL parameter.
     When using `order_by=similarity`, the `sort` parameter is ignored.
     When the `search` parameter is not provided, the API returns the projects ordered by `name`.

• https://docs.gitlab.com/ee/user/group/import/#migrated-project-items

  Footnotes:
  1. Imported branches respect the [default branch protection settings](../../project/protected_branches.md)
     of the destination group, which can cause an unprotected branch to be imported as protected.

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