Break out docs CI job to its own github action

[alamb]

Which issue does this PR close?

Re #2149

Rationale for this change

Running all CI jobs on all PRs is wasteful; Breaking the workflows into multiple actions is a precursor to being more deliberate about what actions to run based on changes in this PR

What changes are included in this PR?

Break the rustdoc check into its own file, name it reasonably

Are there any user-facing changes?

[alamb]

Example of the docs job running: https://github.com/apache/arrow-rs/runs/7483565939?check_suite_focus=true

[codecov-commenter]

Codecov Report

Merging #2151 (e4ce4b9) into master (19a7189) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2151   +/-   ##
=======================================
  Coverage   82.86%   82.86%           
=======================================
  Files         237      237           
  Lines       61296    61296           
=======================================
  Hits        50792    50792           
  Misses      10504    10504           

Impacted Files	Coverage Δ
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us.

[ursabot]

Benchmark runs are scheduled for baseline = fce6626 and contender = 1621c71. 1621c71 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java