Clarify MathJax instructions in mod_astra

[markkuriekkinen]

In the aplus-manual, improve the explanation of using MathJax.js for mathematics in the Moodle plugin mod_astra.

• https://github.com/apluslms/mooc-grader/blob/master/doc/README_Moodle_frontend.md
• in A+, MathJax.js can be included in the _templates/layout.html file with the data-aplus attribute so that the JS is included in chapters.
• In mod_astra, this can't be done since Moodle already includes its version of MathJax and MathJax breaks when it is included twice in the page. Either the script is not included in _templates/layout.html or at least the data-aplus attribute must be left out.
• In Moodle courses, the same course git repo can be used in A+ for testing if the MathJax.js is included in the A+ course settings (head URLs). (Because Mathjax can't be included directly in the chapters due to Moodle.)