Port CSS/js for semantic highlighting and section highlighting.

[csyonghe]

For example, clicking on the link of "T" on https://shader-slang.org/stdlib-reference/types/array-0/getcount-3
should jump back to the doc page of Array and highlight the T parameter:

This is lost in readthedocs.

Also the readthedocs rendering loses all semantic highlighting:

Compare to the current stdlib reference:

The font size of different heading levels doesn't look good and should adjust to similar relative scale to our current doc.

[aidanfnv]

The jumping does work, see the current build of the same page on readthedocs.
Highlighting will need to be ported over though. EDIT: I misunderstood; this issue is about the highlighting not being present for that jumping.