Docs: Add `Separate mobile URLs` subsection

Inform users that, if they use separate URLs for desktop and mobile
users, they should consider helping search engine algorithms better
understand the configuration on their web site.

See also:

 * https://developers.google.com/webmasters/smartphone-sites/details#separateurls
 * https://developers.google.com/webmasters/smartphone-sites/feature-phones

Close #1504.