doc: Update tool sets documentation for modern FreeBSD

doc: Update tool sets documentation for modern FreeBSD

For several operating systems, we give handy package manager one-liners
to install all the requirements for building our documentation.  All
current production FreeBSD releases have a friendly new package manager
a bit like apt/yum, so give a one line command here.  Also, add a brief
note about gmake vs make in the doc subdirectory.

Author: Thomas Munro <thomas.munro@enterprisedb.com>