Add links to Discord on the website (#8343)

* Add link to Discord to index.rst.
* Add link to Discord to contributing.md.

Author: svekars

docs/source/contributing.md

@@ -9,3 +9,5 @@ The source is hosted on GitHub at
 See
 [CONTRIBUTING.md](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md)
 for detailed information about contributing to ExecuTorch.
+
+Join us on [Discord](https://discord.com/invite/Dh43CKSAdc)!

docs/source/index.rst

@@ -31,7 +31,10 @@ Welcome to the ExecuTorch Documentation
    </div>
 
 The ExecuTorch source is hosted on GitHub at
-https://github.com/pytorch/executorch.
+https://github.com/pytorch/executorch. 
+
+Join us on `Discord <https://discord.com/invite/Dh43CKSAdc>`__ if you have questions
+about ExecuTorch or would like to become a contributor!
 
 Getting Started
 ~~~~~~~~~~~~~~~