Give additional guidance on Github Flavored Markdown

[rohanmahy]

From timo:

Using https://github.github.com/gfm/ as a basis, I have identified the following problems with GFM:

• Some useful features are extensions: If we go with GFM, we should at least specify what extensions to use
• We do not want syntax for inline media: Instead, use MIMI MultiPart
• We do not want syntax for raw html: This increases the scope a lot, since GFM does not define a whitelist. Clients would also need to impose additional security constraints.
• No smart timestamps that respect timezones (see https://discord.com/developers/docs/reference#message-formatting-timestamp-styles )
• No built-in interactive elements like polls.
• "Soft line breaks" may be ignored (decided by renderer): If the user typed a newline, they probably want a newline
• Ordered lists must increment by 1 and strip leading zeros 001 -> 1: Markup languages should not change the meaning of the input
• No spoiler syntax for messages or media: In Discord it is ||secret||
• No autolinks for mimi://
• Emojis can have different meanings between platforms, like gun vs water pistol: Maybe clients can annotate what emoji style they use?

I also thought about math support, but I think that should not be part of the text message and instead be a different MIMI MultiPart entry of another content type.

[rohanmahy]

The emoji issue is clear. Unicode describes the semantics of an emoji, not it's presentation.