Inline Olm & Megolm specifications

[Johennes]

This inlines the Olm and Megolm specs into Matrix spec as suggested in #1543. It does this by:

• Adding a new top-level section
• Defining \[ ... \] and \( ... \) as passthrough delimiters for block and inline math expressions
• Calling transform.ToMath in a passthrough render hook to convert Latex to MathML with additional Katex styling
• Bundling the https://github.com/KaTeX/KaTeX CSS and fonts with the spec to avoid loading external resources
• Allowing unsafe-inline as style-src in CSP. This might be a deal breaker but I couldn't find another way around it so far. transform.ToMath will add a myriad of inline styles for vertical positioning without which many formulas are illegible.
• Adding nohighlight as language on any untyped code block. This is necessary because Docsy itself will otherwise enable its own Katex integration which is not CSS-only.

Things to figure out:

• Whether we want to do this at all
  • Best discussed in Olm and Megolm should be specced in the actual spec #1543.
• Placement within the spec
  • Top-level chapter – this is what the PR currently does
  • Appendix subsection
  • Entirely separate page (might require extracting a common theme)
  • ?
• How to best enable rendering of math expressions without CSP issues
  • transform.ToMath – this is what the PR currently does while relaxing CSP rules
  • Rendering math on the client-side using JavaScript
  • Pure MathML (would probably be rather hard to maintain)
  • ?

Fixes: #1543

Pull Request Checklist

• Pull request includes a changelog file
• Pull request includes a sign off
• Pull request is classified as 'other changes'

Preview: https://pr2226--matrix-spec-previews.netlify.app