Document Markdown table syntax

[wbamberg]

This PR is to document the decisions about writing tables in Markdown from #4325.

I also moved up the "Extensions" headings to H2 so they are reachable from the ToC. Why doesn't the ToC list H3 headings?

I also ended up removing the bit that talks about interesting things GFM supports (tables and code fences/info strings) because that sits better in the sections that make use of these features.

[github-actions]

Preview URLs

• https://pr4718.content.dev.mdn.mozit.cloud/en-US/docs/MDN/Contribute/Markdown_in_MDN

Flaws

None! 🎉

External URLs

URL: /en-US/docs/MDN/Contribute/Markdown_in_MDN
Title: Markdown in MDN
on GitHub

• https://github.github.com/gfm/ (2 times)
• https://spec.commonmark.org/ (1 time)

[Ryuno-Ki]

GitHub is showing me an unverified commit (in case that would block merge once the review passes).

[Elchi3]

Looks great, thanks @wbamberg 👍

I think the unverified commit is no problem here but it might be good if Will fixes it at some point :) It is required for yari afaik.

[wbamberg]

Yeah I know. Somewhere along the way GitHub forgot who I am.