Change documentation references to PG website to use https: not http:

Change documentation references to PG website to use https: not http:

This is more secure, and saves a redirect since we no longer accept
plain HTTP connections on the website.

References in code comments should probably be updated too, but
that doesn't seem to need back-patching, whereas this does.

Also, in the 9.2 branch, remove suggestion that you can get the
source code via FTP, since that service will be shut down soon.

Daniel Gustafsson, with a few additional changes by me

Discussion: https://postgr.es/m/9A2C89A7-0BB8-41A8-B288-8B7BD09D7D44@yesql.se