Docs: Add CNAME file to static site source

Summary:
Add a `CNAME` to the docusaurus static docs root so that it's added/retained at the root of the `gh-pages` branch on `docusaurus deploy`, as recommended in https://docusaurus.io/docs/deployment#github-pages-overview

Without this, a [deploy commit will delete](f61db11#diff-76b26edb75578663ca0fd6515ec7880ca04046bdbab298938981c9f0bd3907dd) any existing `CNAME`, which is required for GitHub pages custom domains.

Reviewed By: motiz88

Differential Revision: D52330801

fbshipit-source-id: 577340bfb9e7ec2b23ad78baefdf4eb1099d4ccc

website/static/CNAME

@@ -0,0 +1 @@
+metrobundler.dev