Add supported markdown guide to creating announcement UI

[notbakaneko]

Adds a button

that pops up a guide for the markdown that is supported

Not sure it should be a wiki page instead or pull markdown from somewhere else (or if this kind of input guide would be useful in other places) 🤔
Draft because this is largely a PoC 👀

Could use some ideas for the formatting of the display @arflyte

#9741

[RockRoller01]

Some things are missing from this guide:

• bold emphasis
• underscore based syntax