docs/schedulers: add runopts to documentation

[d4l3k]

This adds the runopts to the documentation for the schedulers. It uses a sphinx plugin to load the scheduler and generate the CLI help text which it places in a code block.

Test plan:

make clean html

[facebook-github-bot]

@d4l3k has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[codecov]

Codecov Report

Merging #374 (45d0e70) into main (92e6897) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #374   +/-   ##
=======================================
  Coverage   94.66%   94.66%           
=======================================
  Files          61       61           
  Lines        3208     3208           
=======================================
  Hits         3037     3037           
  Misses        171      171           

Impacted Files	Coverage Δ
torchx/schedulers/kubernetes_scheduler.py	94.41% <ø> (ø)
torchx/schedulers/local_scheduler.py	93.25% <ø> (ø)
torchx/schedulers/slurm_scheduler.py	96.94% <ø> (ø)

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