Remove references to dev branch in preparation for deprecation

[jdangerx]

Overview

Part of #3179.

Remove references to dev branch, since we'll be using main/stable/nightly instead.

• in git workflow docs (replaced with main)
• in readthedocs URLs (replaced with /stable/)
• in GHA workflows (replaced with main)
• in gcp_pudl_etl.sh (replaced with main)
• in datasette template (replaced with main, but maybe should point to latest or nightly?)

If the builds from #3164 pass, and the nightly branch gets updated, we will merge this as the final PR into dev and lock the dev branch. Then we'll force main to point at the tip of dev and all future development will work from there.

Testing

We need to see if the nightly builds do successfully build on main instead of dev after we do all the finagling above. We can't use a tag build to test that before we merge this PR, because a lot of the stuff we're testing is specific to main, but we can kick off a tag build immediately after we move main.

To-do list

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1. Review the PR yourself and call out any questions or issues you have

[zaneselvans]

We'll also need to change the dependabot and pre-commit-ci target branch from dev to main