Support attribution and license details / configuration

[ES-Alexander]

Adidoks supports some extra content in a footer at the bottom of pages, which we currently use for attribution:

It'd be nice if we supported similar content in a section at the bottom of the BlueTheme sidebar or content section.

I think there should be an [extra.sidebar] config section for this with attribution, docs_license, and code_license variables (kind of similar to the AdiDoks footer variables), with default values (or implementation defaults) of

variable	value
attribution	Powered by Zola and BlueTheme
code_license
docs_license	Documentation under CC BY-NC-SA 4.0
project_owner