[WIP] Markdown guide

[redgoldlace]

Summary

Adds a guide-level explanation of Discord's markdown subset. This is very much a work in progress and currently contains a few CSS/RST hacks that I'm not happy with. Likewise the content isn't entirely fleshed out yet, and some sections are still missing.

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [x] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] This PR is not a code change (e.g. documentation, README, ...)