Add fsspec to Read The Docs requirement (#536)

Currently the Read The Docs pages for this repo aren't providing the
detailed information for the functions (only the landing page is working
as expected). The Read The Docs build says that it succeeded but gives
errors about not being able able to find the `fsspec` module, see the
[log](https://app.readthedocs.org/api/v2/build/27889860.txt) for more
detail.

I've added `fsspec` to `docs/requirements.txt` to fix this issue.

Author: briangow

Diff for: docs/requirements.txt

@@ -2,3 +2,4 @@ numpydoc==1.7.0
 sphinx==7.0.0
 sphinx_rtd_theme==3.0.0
 readthedocs-sphinx-search==0.3.2
+fsspec>=2023.10.0