New Contributor Guide documentation for newcomers to ExecuTorch or open-source

[pytorchbot]

Summary

New Contributor Guide for people new to ExecuTorch, GitHub and/or open-source. Submitting PR as requested in the #executorch-just-works Discord channel.

Note: May need to confirm the github-action release notes label point discussed in the guide - I don't have permission to assign labels, but this may be a permissions issue rather than intentional.

Todos For Later

As discussed in the comments, we'd like to make the following additions to this guide over time (not in this PR):

• Build Instructions: Since the build process is not detailed here, consider providing a brief overview or common pitfalls users might encounter. This could reduce the number of questions on your Discord server.

• Review Process: Clarify the expected timeline for reviews and what contributors should do if they don't receive feedback within a certain period.

• Common Issues: Consider adding a FAQ or common troubleshooting section for issues that new contributors frequently encounter.

  • Draft FAQ intro text:

    ## FAQ
    
    Some answers to some frequently asked questions are given below. If you still can't find what you're looking for, please feel free to:
    * ask on our [Discord Server](https://discord.com/invite/Dh43CKSAdc).
    * search or post on our [GitHub Discussions forum](https://github.com/pytorch/executorch/discussions).
    
    [Insert actual FAQ questions and answers here.]
    

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/10110

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 3 Pending

As of commit a91ead7 with merge base 771588a ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm / linux-job (gh)
  RuntimeError: Command docker exec -t 63dcb23431f57364edcdb2e0ea4aafdb9e2daee894bc6dd07cdb75f3faf61d7c /exec failed with exit code 1

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