Doc build fixes: pin pydata theme version, pre execute ipynbs

[orionlee]

1. Pin pydata theme to v0.8.1 due to issues with the latest version.
2. Provide a way to pre-execute ipynbs for doc build, to avoid the problem of intermittent errors from MAST breaking the build. See the instructions in docs/pre_execute_ipynbs.py for detials.

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Issues with the latest pydata theme v0.12.0

• @christinahedges reported that searchbar on the left is missing
• the way to specify "google_analytics_id" is changed. See https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/analytics.html
• The latest, v0.12.0, does not work with the latest sphinx (v6.x), either. See https://github.com/pydata/pydata-sphinx-theme/issues/1094

Pinning pydata theme to v0.8.1 (and indirectly the sphinx 4.5.0) to temporarily work around the problem.

[orionlee]

This PR is based on v2.4.0 tag , thus creating some conflicts with main. Let me know if you'd prefer I rebase the PR to main.

[christinahedges]

Hi @orionlee I made some edits to your PR just to add some log messages to make it easier to see how much the script has to be rerun. I also added it in a makefile and I'm going to see if this does make it easier to run our notebook/docs.

It does look like I messed up the rebase though and I got myself a bit confused. Sorry I've made a mess here I'll try to clean it up!