[markdown] Document markdown idiosyncrasies

What does this MR do and why?

There are several idiosyncrasies of markdown formatting and rendering which generate questions and issues. These should be documented somewhere so we can point to the same answer.

Such as

• !81079 (comment 853258091) - Creating sublists within order lists vs unordered lists provides a different experience/output
• !81079 (comment 853265373) - Placing 2 unordered lists (including task lists) in a row does not render properly
• https://gitlab.slack.com/archives/CETG54GQ0/p1645654864161539 - newlines seem to cause weird spaced formatting in markdown lists

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.