New Contributor Guide documentation for newcomers to ExecuTorch or open-source (#10110)

### 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.]
    ```

Co-authored-by: jhelsby <[email protected]>

Author: pytorchbot

CONTRIBUTING.md

@@ -100,6 +100,8 @@ executorch
 ## Contributing workflow
 We actively welcome your pull requests (PRs).
 
+If you're completely new to open-source projects, GitHub, or ExecuTorch, please see our [New Contributor Guide](./docs/source/new-contributor-guide.md) for a step-by-step walkthrough on making your first contribution. Otherwise, read on.
+
 1. [Claim an issue](#claiming-issues), if present, before starting work. If an
    issue doesn't cover the work you plan to do, consider creating one to provide
    context about it, and to build consensus about the scope and solution.