Enable Sphinx math support (`:math:` role) in the docs build

[djolertrk]

docs/conf.py does not enable any Sphinx math extension, so the :math: role does not render and the output shows raw MathJax delimiters instead, e.g. \(-2^{63}\).

Reproduce: add e.g. :math:`2^{63} - 1 to any .rst page and build the docs, so the formula appears as literal \(...\) text.

Fix: add sphinx.ext.mathjax to extensions in docs/conf.py (or sphinx.ext.imgmath if the rendered site should not load the MathJax script from a CDN).

No page currently uses :math:, so in the doc ethdebug page we worked around it with plain literals like 2**63 - 1.

[djolertrk]

cc @cameel

[cameel]

For context: this came up in the ethdebug spec PR argotorg/solidity#16780 (comment)

add sphinx.ext.mathjax to extensions

Before we do that I'd check if it bundles MathJax so that it still works without external JS. It's bad enough that our docs don't even render without JS, but having them also not even build/render offline would be unacceptable.

[clonker]

sphinx.ext.imgmath is probably an option if latex is available on the rtd servers, otherwise i've made good experiences with katex in the past. afaik it can be configured to run offline.