Clarify documentation on Markdown support

[jasonekratz]

Describe your feature request Update Markdown documentation to make it clear that FSNotes isn't just using CommonMark but that it's using the GitHub Flavored Markdown extension of CommonMark so users know exactly what syntax is supported. Right now the wiki here in GitHub talks about how it supports CommonMark then has syntax editions that aren't part of the Markdown spec (like tables). Then there is a "random" GFM document that gets installed with the app. This stuff should be combined and included on the wiki page.

Additional context None