Revise CI configuration

[khaeru]

• Separate docs build from lint.yaml: this step requires installing the package and dependencies, and makes this job a slow one, when it should give immediate feedback on code quality.

• Separate tasks that can run in parallel, i.e.:

  • pytest
  • mypy, docs build.

  Since GHA doesn't limit concurrent workflows/jobs, this should shorten the total execution time for all checks.