dev: Build docs in parallel

[thomaspinder]

The docs currently take ~15mins to build. As GPJax grows, this runtime will likely increase so we should explore options of building the docs in parallel. Through nbsphinx, the -j flag looks like a potential solution; however, we'll need to explore how many cores are available in readthedocs.

Example:
python3 -m sphinx . _build -j4 will build the docs across 4 cores.