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

`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``.

Start in docs/conf.py and inspect the existing extensions list. Reproduce the issue by adding a :math: role such as `2^{63} - 1` to an .rst page, then build the documentation. Done means the formula renders as math rather than literal MathJax delimiters.

Written by the indexing model from the issue text.