update fork URLs for doc build

[matthiasdiener]

https://github.com/pradyunsg/furo/discussions/693

Questions for the review:

• [ ] Is the scope and purpose of the PR clear?
  • [ ] The PR should have a description.
  • [ ] The PR should have a guide if needed (e.g., an ordering).
• [ ] Is every top-level method and class documented? Are things that should be documented actually so?
• [ ] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [ ] Does the implementation do what the docstring claims?
• [ ] Is everything that is implemented covered by tests?
• [ ] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?

[matthiasdiener]

Fixes both the CI build and the RTD build (https://readthedocs.org/projects/mirgecom/builds/21641699/)

[matthiasdiener]

Looks like a fix is incoming: https://github.com/pradyunsg/furo/commit/9b94420776972613c3990a31467855ceceefcff7

[matthiasdiener]

Seems the underlying bug is fixed in sphinx 7.2.2, so this PR only updates a few URLs now.