(#19006) Clarify the search order for templates

This patch updates the templating guide to be consistent with comments in the
Puppet codebase. When resolving a relative path to a template, Puppet will
first search relative to module paths and then fall back to searching relative
to the paths in `templatedir` if no matches are found.
(#19006) Link learning template chapter to guide

Add a cross-reference from the Templates chapter in Learning Puppet to the
Using Templates Guide.