Allow the use of a custom overrides.json file for configuring JupyterLite at runtime

[agriyakhetarpal]

Description

This PR introduces a configuration option in conf.py called jupyterlite_overrides. This option modifies the command used to build JupyterLite internally within jupyterlite-sphinx to use a user-provided overrides.json file.

Rationale

• It is currently possible to leave an overrides.json file in the docs/ directory, similar to ipycanvas's documentation, which will automatically be picked up by jupyterlite-sphinx (and thereby JupyterLite) when building the JupyterLite site.
• However, projects using jupyterlite-sphinx can benefit from the flexibility of using a custom override file with the docs, which is possible if one has access to the JupyterLite build command with ease.
  This option is already available via https://jupyterlite-sphinx.readthedocs.io/en/latest/configuration.html#additional-cli-arguments-for-jupyter-lite-build, which was established via Pass additional configuration options to the jupyter lite build command #169. Still, that approach has been directed mostly towards advanced users, and having this feature exposed through a configuration option similar to one for a build-time configuration file would make it easier to use and make it more visible with the rest of the options in the docs.

Additional context

Please see jupyterlite/jupyterlite#385 (comment), which is where I got this idea. Some possible use cases for this can be:

• to be able to add download buttons for interactive notebooks within SciPy and PyWavelets that have been/are being established via DOC: stats: Convert sampling tutorial to MyST-md scipy/scipy#20303 (cc: @melissawm) and DOC: Add interactive notebooks to pages in the "Usage Examples" section PyWavelets/pywt#741
• customise themes based on the surrounding theme of the page (dark or light – not sure how to switch it yet, but we do have an open issue about it: Provide a theme toggle plugin by default #124).

[agriyakhetarpal]

I've (temporarily) pushed bf5e616, which enables the https://github.com/Quansight-Labs/jupyterlab-theme-christmas theme and a download button for notebooks within our documentation. It is working nicely and as expected, and the changes can be tried out in the PR preview: https://jupyterlite-sphinx--225.org.readthedocs.build/en/225/

[agriyakhetarpal]

Rebased on top of main and removed the Christmas theme I had added for experimentation. I can confirm that it continues to work with a local build. Thanks again for the review, @Carreau.