[ANN] Dropping MathJax 2 support in Org 9.6

[Ihor Radchenko]

Dear Orgers,

We are moving away from the outdated MathJax 2 to MathJax 3 in Org export.
This change is a breaking change as MathJax 2 is not compatible with
MathJax 3+.

We have done all we could to not break the existing usage. However, some
users might still be affected. In particular, people who customized
`org-html-mathjax-options' to point to a custom MathJax 'path to MathJax
2 will experience breakage. Other users, including people who customized
other MathJax options (not 'path), will _not_ be affected as Org will
auto-convert MathJax 2 style options to MathJax 3.

See more context and the discussion in
https://list.orgmode.org/orgmode/m2a667n4ax.fsf@me.com/

Commit: 
https://git.savannah.gnu.org/cgit/emacs/org-mode.git/commit/?id=62e1513b5

-------------------------------------
Below is the relevant ORG-NEWS entry:

Org now uses MathJax 3 by default instead of MathJax 2.  During HTML
exports, Org automatically converts all legacy MathJax 2 options to
the corresponding MathJax 3+ options, except for the ~path~ option in
which now /must/ point to a file containing MathJax version 3 or
later.  The new Org does /not/ work with the legacy MathJax 2.

Further, if you need to use a non-default ~font~ or ~linebreaks~ (now
~overflow~), then the ~path~ must point to MathJax 3 or later.

See the updated ~org-html-mathjax-options~ for more details.

MathJax 3, a ground-up rewrite of MathJax 2 came out in 2019.  The new
version brings modularity, better and faster rendering, improved LaTeX
support, and more.

For more information about new features, see:

https://docs.mathjax.org/en/latest/upgrading/whats-new-3.0.html
https://docs.mathjax.org/en/latest/upgrading/whats-new-3.1.html
https://docs.mathjax.org/en/latest/upgrading/whats-new-3.2.html

MathJax 3 comes with useful extensions.  For instance, you can typeset
calculus with the ~physics~ extension or chemistry with the ~mhchem~
extension, like in LaTeX.

Note that the Org manual does not discuss loading of MathJax
extensions via ~+HTML_MATHJAX~ anymore.  It has never worked anyway.
To actually load extensions, consult the official documentation:

https://docs.mathjax.org/en/latest/input/tex/extensions.html

Lastly, MathJax 3 changed the default JavaScript content delivery
network (CDN) provider from CloudFlare to jsDelivr.  You can find the
new terms of service, including the privacy policy, at
https://www.jsdelivr.com/terms.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>