Do not use shallow clones in Read the Docs to properly show last upda…

[mathbunnyru]

…ted time

Describe your changes

Issue ticket if applicable

Checklist (especially for first-time contributors)

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests
• I will try not to use force-push to make the review process easier for reviewers
• I have updated the documentation for significant changes

[mathbunnyru]

We use sphinx-last-updated-by-git which has a caveat described here: https:/mgeier/sphinx-last-updated-by-git

I checked and the fix works.
Before: https://jupyter-docker-stacks.readthedocs.io/en/latest/maintaining/new-images-and-packages-policy.html

After: https://jupyter-docker-stacks--2157.org.readthedocs.build/en/2157/maintaining/new-images-and-packages-policy.html