Allow escaped characters to be used inside markdown math

What does this MR do and why?

Certain characters, such as $, %, #, &, {, }, and _, must be escaped when used in LaTeX math blocks (https://katex.org/docs/support_table.html#symbols). This MR just targets the ones listed above. Additional support will be added with #387585

In addition, the handling of the $$/n.../n$$ syntax before passing it to markdown fixes a variety problems for those math blocks.

How to set up and validate locally

1. Visit an issue, and either edit the description or add a comment

2. Add

   $1 \& \$2$

3. When previewing, you should see

4. Add

   $$
   \dot{X} = \bold{\frac{A}{\rho B}}_{\text{first term}} + \bold{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \bold{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}
   $$

   

5. Add

   $$
   \dot{X} = \underbrace{\frac{A}{\rho B}}_{\text{first term}} + \underbrace{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \underbrace{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}
   $$

   

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #373162 (closed)