Add CODE_OF_CONDUCT.md (#4239)

This PR adds a `CODE_OF_CONDUCT.md` that links out to the existing code
of conduct on the Zed site.

The link in the file is currently a dead link, but will work once the
new site goes live tomorrow.

Release Notes:

- N/A

CODE_OF_CONDUCT.md

@@ -0,0 +1,3 @@
+# Code of Conduct
+
+The Code of Conduct for this repository can be found online at [zed.dev/docs/code-of-conduct](https://zed.dev/docs/code-of-conduct).