Serving CORS headers for fonts assets

Rails has a feature to simply configure a CDN to the application.

When this feature is used, CORS headers need to be served to client browser for the font assets. So far we've been using `font_assets` gem but it is obsolete since a long time. Switch to using `rack-cors` gem and add ability to configure CORS for other purposes as well.

The need for documentation is because in the past with `font_assets` gem we had CORS working by default. Now the configuration file `config/examples/cors.yml` needs to be copied to the `config/` directory for CORS to work for fonts.

If any customers used a CDN, then an upgrade with this change will cause CORS headers to NOT be sent thus fonts will not be able to be downloaded.

On the other hand this is not a supported configuration so documentation should not suggest that I guess. Or should it be now a supported configuration? hramihaj, mcassola?