Switch from dev to main/nightly/stable branch structure.

[zaneselvans]

Overview

• Update docs and build scripts to reflect new branch structure.
• Run builds every night, but only if there are changes.
• Rename nightly-build-outputs.catalyst.coop bucket to builds.catalyst.coop.
• Remove docker/.env file and define those envvars elsewhere.
• Add some basic release notes about our new branches / workflow

Closes #3140
Closes #3179

Testing

Ran a test build on the branch before it got merged into dev and all went well. The real test will be... do the nightly builds run the way we think they should.

To-do list

Beta Give feedback

1. Make sure full ETL runs & make pytest-integration-full passes locally
2. For major data coverage & analysis changes, run data validation tests
3. If updating analyses or data processing functions: make sure to update or write data validation tests
4. Update the release notes: reference the PR and related issues.
5. Review the PR yourself and call out any questions or issues you have

[zaneselvans]

Goodbye dev 😭