docs: document the BBCode, Embed, Emoji and Markdown extensions

[karl-bullock]

Adds admin pages for four bundled extensions that had none, as agreed with Ian: BBCode, Embed, Emoji and Markdown. Each is added to the Bundled Extensions sidebar in alphabetical order.

• Markdown: the supported syntax (including superscript, subscript and both spoiler forms), the toolbar, the Ctrl/⌘+B and Ctrl/⌘+I shortcuts, and list continuation on Enter. Notes that tables aren't supported and that code highlighting loads from jsDelivr.
• BBCode: all supported tags with examples.
• Emoji: the : autocomplete and toolbar button, the emoticon list, and the CDN mirror setting. Notes that a typed :rocket: is never converted.
• Embed: enabling it, the embed URL format, the host-page snippet with iframe-resizer v4, and what readers see. Also covers logging in from inside the embed: it works on the same site, but cross-site embeds stay logged out unless cookie.samesite is set to none.
• Every page: what happens to existing posts when the extension is disabled. For Markdown, BBCode and Emoji, formatting stops showing and comes back when the extension is re-enabled.

Everything was checked on a stock 2.0.0 forum: posts with every syntax, rendered HTML read back through the API, each extension disabled and re-enabled, and a discussion embedded in a host page with iframe-resizer.

Not included:

• Images: the pages have no hero or share images yet. I'm assuming you'll add them to match the other extension pages.
• ?hideFirstPost=1: left out of the Embed page because it doesn't work on 2.0.0 or 2.x; reported as [2.x] Embed's hideFirstPost option doesn't hide the first post framework#5167.

The composer.json docs links are in flarum/framework#5168.

[imorland]

Thanks Karl. I added the images. Good to go!