Add mathjax to properly render mathjax content

[benoit74]

Fix #9

Changes:

• install MathJax from sources on scraper side (failed to work properly on JS side since we want to include it dynamically)
• setup MathJax based on libretexts.org configuration / needs
• add logic to reload MathJax everytime we change page since we need to compute current page front, used to properly number equations / figures / ...
• reorder processing steps to do "simple and fast" things first
• adapt CONTRIBUTING.md
• also add page title to display

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.

Project coverage is 50.72%. Comparing base (ff4240b) to head (2f0a3ef). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/libretexts2zim/processor.py	0.00%	18 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #31 +/- ## ========================================== - Coverage 51.37% 50.72% -0.65% ========================================== Files 9 9 Lines 547 554 +7 Branches 61 63 +2 ========================================== Hits 281 281 - Misses 262 269 +7 Partials 4 4 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[benoit74]

For the record, I've moved from a directive to a service, since this indeed seems more appropriate / simple (and probably more maintainable).