Add documentation

as the module stabilizes, we should add some documentation. this could be a good place to share recipes on how to solve different use cases with facet api pretty paths.

documentation is drupal is a bit unstructured. i think we should add a child-page next to facet api in the Search modules section. we'd want a general "start page" on how to install and use the module + sub-pages with use case descriptions below.

coming from #1922086: Add support for base path aliases, also see Online documentation structure and some existing documentation references on the module page.