[lldb][Docs] Convert GDB protocol extensions doc to Markdown and add to website (#89718)

This document has never been on the website, unlike GDB's protocol docs.
It will be useful to have both available online to compare.

Markdown is easier to edit and preview in many editors (including Github
itself), so I've chosen that over RST. Plus, building the website takes
minutes and I lose the will to make nice edits when I have to deal with
that.

The standard dialiect lacks some things notably multi-line table cells,
so I've converted large tables into bullet point lists
so that we still get text wrapping. This is a downside but I think the
simplicity of Markdown outweighs this.

I have applied the plain text markers where I've noticed it and escaped
some HTML characters. There may be more changes needed but, it's
Markdown, so it's in theory a lot easier for someone to fix it!