Correctly describe GLFM features

What does this MR do?

Reviews https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/user/markdown.md for accuracy.

Specifically, addresses inaccuracies in how describing features not present in standard Markdown: #432869 (comment 1704512565).

Also:

• Makes section Differences between GitLab Flavored Markdown and standard Markdown the SSoT for differences between the different Markdowns.
  • Deletes sentences like "This is extended from standard Markdown" from individual features' sections.
• Deletes sections Features not found in standard Markdown and Features extended from standard Markdown and de-indents their subsections. This way, the most deeply nested heading is h4, not h5.

TOC comparison 🖼

Before	After

This MR doesn't:

• Move the sections that were previously under Features not found in standard Markdown and Features extended from standard Markdown to keep this MR's scope to improving the page's accuracy, not style.

Related issues

• Review and update GLFM doc: features extended f... (#432869 - closed)
• #296296 (comment 1455639433)
• https://gitlab.com/gitlab-org/technical-writing/-/issues/936+

The regular good stuff

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