Properly apply MathJax configuration

[benoit74]

Fix #283

Changes:

• push configuration inline instead of an external script, since only inline configuration is supported, at least in MathJax 2 which we are using
• use python:3.8-buster instead of python:3.8-slim-buster because we need gcc to compile pyzstd wheel (don't get how it worked in the past, maybe pyzstd was available?)

Nota: this is not yet tested due to #322 and archive.org still not being fully back online

[benoit74]

I confirm this works as intended, fixing Mathjax display:

[rgaudin]

👍