Add TypeDoc and Action for docs creation

[zorkow]

This PR adds

• The typedoc package and its configuration
• An action called docs.yml that generates the code documentation on pushing/merging to master branch.

For resulting documentation see here: https://mathjax.github.io/MathJax-src/

I have not yet worked out how to add a favicon.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (028a0f9).
Report is 11 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1299     +/-   ##
==========================================
  Coverage    86.71%   86.71%             
==========================================
  Files          337      337             
  Lines        83979    83979             
  Branches      4750     3124   -1626     
==========================================
  Hits         72826    72826             
- Misses       11130    11153     +23     
+ Partials        23        0     -23     

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[dpvc]

This looks good. It may prompt us to make more descriptive jsDoc comments, even!

I'm wondering if we could use something similar in the MathJax repo to house the current master branch's documentation, with Mathjax-src having the current develop version?