[ci] Split formatting and docs checks into separate task

[jackkoenig]

Now that #2341 is merged, the Scala 2.12 CI takes 17 minutes and the 2.13 CI takes 12 minutes. The 2.12 CI runs documentation and formatting checks, so this splits those out to speed up the critical CI path by a ~3 minutes.

Contributor Checklist

• [NA] Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• [NA] Did you add appropriate documentation in docs/src?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• [NA] Did you add text to be included in the Release Notes for this change?

Type of Improvement

• ci

API Impact

No impact

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

[jackkoenig]

This reduced total CI time from 17m47s [1] to 14m26s [2].