Use MathJax and incorporate `preamble.pty`

[kory33]

Currently, the site uses KaTeX and we want to use MathJax to

• align the environment with my local Obsidian
• use macros to define symbols

To do this, we need to

• modify the header file and the included partial header file
• allow MathJax to load preamble.pty. We need to do two things:
  • in GitHub Actions script, somehow throw in preamble.pty to the build artifact, and then
  • edit the template file to load preamble.pty as the published preamble

[kory33]

Commits 634491f...00e35a4 resolved this, although I am experiencing some minor issues.