Add notice for Google searchers

[wsargent]

Searching for Play documentation presents some problems for experienced programmers: they use Google, and following the usual route of "search terms web framework" doesn't work for Play, because it shows old versions or Google Play.

• https://twitter.com/kelleyrobinson/status/975376400025903104
• https://www.reddit.com/r/scala/comments/853v7q/play_rantframework_suggestion/

The best way to drill down into Play documentation is to use "site:playframework.com 2.6.x" on to the end of the google query, but that doesn't get exposed to people.

Looking at the "Referer" header to see if it includes "google" and then adding a modal dialog that says "Hi, we noticed you arrived on this page from Google, did you know adding site:playframework.com 2.6.x to your query will help find what you're looking for more easily?" may help.

[marcospereira]

I'm not an SEO specialist, but maybe we can use the sitemap.xml to lower the priority of Play 1 pages. Given the fact that we already have canonical URLs (curl -I https://www.playframework.com/documentation/2.6.x/Home), this should bring the most current docs up.

When combined with #200, this should improve the experience of people searching for Play docs.