OKR: Improve the Markdown page (part 1)

What does this MR do?

Page to be improved: https://docs.gitlab.com/ee/user/markdown.html
OKR issue: #205689 (closed)

Related issues

1. Closes gitlab-docs#652 (closed)
   Update the footnotes section to have the example footnote on one line. This appears correctly in GitLab, but doesn't work properly in docs.gitlab.com.
2. Closes #24731 (closed)
   Document using [[_TOC_]] in Wikis.
3. Closes #20612 (closed)
   Add a mention that `code` in deletions or additions needs to have escaped backticks.

Related to #205689 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.