MathJax library has been loaded via "defer" method since v4.3.0.  But
it prevents to work MathJax v2.  This rollbacks the change and use
"async" method as default again.

To support changing the loading method, this allows to specify it via
mathjax_options.

To allow configure MathJax via static JS file, it should be loaded via
"defer" strategy.

This new method in the `Sphinx` object allows extensions to communicate to
Sphinx that it's preferred to include HTML assets in all the pages. However,
it's extensions developers' responsability to follow this config and decide
whether or not include the assets required.

Extensions developers' can check `Sphinx.html_assets_in_all_pages` together with
any other logic they may have to decide if the assets will be included in the
rendered page or not.

Closes #9115

Allow to add JS/CSS files to the specific page when an extension calls
`app.add_js_file()` or `app.add_css_file()` on `html-page-context`
event.

.. note::

   $ find sphinx tests LICENSE doc/conf.py -type f -exec sed -i '' -e 's/2007\-20../2007-2021/' {} \;
   $ git co sphinx/locale/**/*.js sphinx/templates/epub3/mimetype