pick_doc_commits.py prints commits we should cherry-pick (#863)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/863

Helps find commits to cherrypick into a release branch.

Usage:
  pick_doc_commits.py --main=origin/main --release=origin/release/5.5

It will find commits on the main branch that are not on the release branch, and
filter them down to the docs-only commits that should be cherrypicked. It will
also print the commits that were filtered out.

This tool will not actually modify the git repo, it will only print the commands
to run.

Must be run from inside the repo, ideally after a recent `git pull`. Does not
care which branch is currently checked out.

Reviewed By: kirklandsign

Differential Revision: D50213694

fbshipit-source-id: e4b20953759b633660a4f386e08383e5ca04eb14