Update cheat sheet to reflect all the supported Markdown markup
I am aware that Nim Forum supports formatting links in Markdown style like `[foo](link)`, but the [current cheatsheet](https://forum.nim-lang.org/about/rst) [[RST Source](https://github.com/nim-lang/nimforum/blob/a1601b4600c7d96dd85e30d05a96d5c41d64b538/public/rst.rst)] 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`](https://github.com/nim-lang/Nim/blob/51ab7ccec1ffa21cef67f1a75dea889653751cfc/tests/stdlib/trstgen.nim) test.

[Discussion on Discord](https://discord.com/channels/371759389889003530/371759389889003532/851842807915741215)