Update cheat sheet to reflect all the supported Markdown markup

[kaushalmodi]

I am aware that Nim Forum supports formatting links in Markdown style like [foo](link), but the current cheatsheet [RST Source] does not show that.

That link format is just one thing I discovered, but there could be more that needs to be documented in that cheat sheet, based on this latest (as of writing this) trstgen.nim test.

Discussion on Discord

[pietroppeter]

I also noticed there is an extended documentation that will come out in 1.6 (currently in devel):

• https://github.com/nim-lang/Nim/blob/devel/lib/packages/docutils/rst.nim#L1
• https://nim-lang.github.io/Nim/rst.html (is this the correct link to devel docs? I was looking for it and found this...)