Add top level CNAME file pointing to a custom domain.

Details:
- This should fix `quarto render` (both local and in CI) which removes
  `CNAME` file if it was added to the rendered site manually (like via
  GitHub Pages settings page which creates an extra commit in `gh-pages`
  branch).

  The 'CNAME' file is recognized automatically as a site resource
  (without having to update '_quarto.yml' to explicitly include it)
  and is copied to publish directory ('docs').

Author: echasnovski

CNAME

@@ -0,0 +1 @@
+nvim-mini.org