Conditionally disabled custom 404 pages on dev docs

[agjohnson]

This is for testing error pages more in production and can be reverted later.

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--11596.org.readthedocs.build/en/11596/

• Developer's documentation (dev): https://dev--11596.org.readthedocs.build/en/11596/

[agjohnson]

I was going to test these in PR build, but readthedocs.build is pointing to our web scaling group. This PR only needs to be in place for a short while either way.

[ericholscher]

PR builds would be another good one to point at the new serving -- low risk 👍

[agjohnson]

Test failures seem unrelated, I'll merge this and we can continue pushing forward on testing with more domains later

[humitos]

PR builds would be another good one to point at the new serving -- low risk 👍

I opened https://github.com/readthedocs/readthedocs-ops/issues/1533 and added to this sprint.