feat(macros): Rewrite MathML sidebar

[wbamberg]

Summary

This PR rewrites the current MathML sidebar, MathMLRef.ejs.

Problem

The immediate reason for this is that the current sidebar uses tags, and we want to stop using tags.

But the current sidebar is also not very useful: it only lists elements, not attributes or any guide/tutorial content.

Solution

This sidebar is pretty much cloned from the PR to rewrite the SVG sidebar, but the content is a little different:

• Guides
• Reference
  • Elements
  • Global attributes
  • Attributes reference
• Examples

Some notes and things.

• There's definitely scope to have a similar organization and ordering at the top level for many of our sidebars (i.e Tutorials/Guides/Reference or something) maybe based explicitly on Divio. There are definitely some pointless inconsistencies at the moment. But that's another story.
• There's also scope for making this sidebar data-driven as we are prototyping in feat(macros): rewrite Learn sidebar #8132. But again, another story.
• This sidebar gets labels from page titles, and some page titles could perhaps be adjusted, like MathML: Deriving the Quadratic Formula -> "Deriving the Quadratic Formula". We will need a follow-up anyway in content to call this sidebar everywhere, so maybe I'll do that then.

Screenshots

Before

After

collapsed:

expanded:

Testing

Testing: fire up yarn dev and visit:

• http://localhost:3000/en-US/docs/Web/MathML
• http://localhost:3000/fr/docs/Web/MathML

@teoli2003 , @hamishwillee , @fred-wang

[teoli2003]

👍

[hamishwillee]

Again, much better than what was there.