Split CI GitHub workflows into 3 files

[hantuzun]

The ci.yaml file could be split into:

• ci-docs.yaml
• ci-lint.yaml
• ci-test.yaml

Then, ci-test.yaml could ignore paths such that PRs only touching docs/** such as #3681 would not need to be tested.

[casey]

I'm fine with that, although, in practice, running the tests takes a lot longer than building the docs or linting, and they happen in parallel, so it doesn't slow down the overall run time.

If we do split them, I'd vote to call them docs.yaml, lint.yaml, and ci.yaml or test.yaml.