Provide way to override/extend settings in pelican.auto.py

[sebbASF]

The pelican.auto.py file is generated from the template bin/pelican.auto.ezt with variables from pelicanconf.yaml.

This is somewhat inflexible - it is not currently possible for sites to add extra settings to the file, nor to change any fixed defaults.

It would be useful to be able to override/extend settings using the peliconconf.yaml file.

[gstein]

This is by design. It is hard to provide a service when "anything is possible". Thus, we provide a limited set of parameters that work for 95% of all users.

[sebbASF]

This will likely become redundant if the builds are moved to GHA, because part of the change as currently envisaged is to replace pelicanconf.yaml with an equivalent pelicanconf.py. This file will then be under the control of the site owner.