Fix intermittent missing fit plots in docs

[shum23]

The interactive fit plots in the docs (e.g. 6_data_fitting.ipynb) would sometimes fail to appear and leave a blank space where the sliders and figure should be. Refreshing the page sometimes fixed it, sometimes didn't. It also seemed to vary by browser.

It turns out both sphinx and bokeh try to load MathJax, and whether this caused a crash depended on which one loaded first. That's why it looked random.

Bokeh 3.9's bundled MathJax uses MathJax 3 API and is still migrating to v4 but Sphinx recently switched its default to MathJax 4 in v9.0, see sphinx.ext.mathjax

Pin mathjax_path to MathJax 3 in docs/source/conf.py as a workaround according to sphinx's docs, matching what bokeh's bundle expects.

Note: Should revisit if/when bokeh adds mathjax 4 support.

Also reduced the embedded states of the fit plots there to make them manageable for docs builds

[phrgab]

There appear to be formatting changes only on file_loaders.md, unrelated to the fixes - is that correct? If so, are you able to drop these changes before we merge?

[shum23]

There appear to be formatting changes only on file_loaders.md, unrelated to the fixes - is that correct? If so, are you able to drop these changes before we merge?

Thanks @phrgab. That's just down to ruff linting. Dropping them would cause the linting CI to fail.