an attempt to fix the Google search problem

It is a well-known problem that a Google search for something in the
PostgreSQL documentation will usually return hits in old documentation
versions first, because those pages have been around for the longest.

I believe I have a promising fix for that. By adding a <link
rel="canonical"> to the documentation pages that point to the "current"
version, search engines will be encouraged to return the current version
search results.

I had heard that the Django project had the same problem and got this
solution from there. See for example the source of this page:
<https://docs.djangoproject.com/en/1.10/topics/db/models/>. Here is
also some information from Google about this:
<https://webmasters.googleblog.com/2013/04/5-common-mistakes-with-relcanonical.html>

I think this is worth trying. A one-line patch is attached.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services