[Doxy] Fetch MathJax from the web rather than vendoring it

[dpiparo]

And add minor fixes on the way

[ferdymercury]

Why exclude core/lzma and core/lz4 and core/newdelete?

I understand why to exclude core/clib (which btw would be no longer explicitly needed with #23040)

Same for core/textinput, in my opinion it should not be excluded, only the part core/textinput/src/textinput (which would no longer need explicit removal with #23046)

[dpiparo]

Thanks for this observation. Some more doc will definitively not break the bank. I think lzma was removed not to expose in the doc of the public interface details related to compression. LZ4 was added at a later stage, and this file was never updated... Or at least that is my theory...

[linev]

@dpiparo

This was too fast - mathjax also used with JSROOT.
See builtins/mathjax/CMakeLists.txt