Fix math formula display (if possible)

[user202729]

See the "test broken math formula" section in https://cryptohack.org/challenges/2d_dlog/solutions/#user202729

For now, some workarounds to write math formulas properly are

• use \lt instead of <

• for display math,

  • always put it in a separate paragraph
  • don't leave any extra space before/after $$
  • always put \\ at the end of the line

  example:

   text.
  
   $$abc \begin{pmatrix} a \\
   b \end{pmatrix} def$$
  
   More text...