Document native Markdown footnote syntax

What does this MR do?

This MR updates the ### Footnotes section of the documentation style guide to recommend native Markdown footnote syntax ([^label] and [^label]: text) instead of hand-written <sup> tags and a manually numbered **Footnotes**: list. It also removes the subsection on working around markdownlint rule MD029 for tables with five or more footnotes, since the new syntax no longer needs it. The rewritten guidance notes that footnotes are numbered page-wide by first reference order, and that a footnote reference inside a shortcode is not processed. Converting existing pages that still use <sup> tags is out of scope for this MR.

Review app

• Preview: https://docs.gitlab.com/upstream-review-mr-ee-255289/development/documentation/styleguide/#footnotes

Related issues

• Tracking issue: Footnote syntax: tracks this style guide update and the still-open conversion of existing pages to the new syntax.
• docs.gitlab.com rendering support (merged): adds native Markdown footnote rendering and moves definitions below the referencing table. Deployed.
• Vale rule fix (merged): stops Vale from flagging [^label]: definitions as reference-style links.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.