Allow for selectively overriding parts of SUSHI's automatic page naming and inclusion

Currently you can either allow SUSHI to name pages for you (based on filename) _or_ you can name them yourself, but then you have to fully specify all pages in `sushi-config.yaml`. Some users would like a way to have SUSHI name most pages for you, but allow you to override names on certain pages. There was also a request to support a way to omit pages.

See: https://chat.fhir.org/#narrow/stream/179252-IG-creation/topic/markdown.20page.20title/near/276401394

Start by tracing SUSHI's automatic page naming and the page configuration described in sushi-config.yaml. The change is done when users can keep automatic naming for most pages while overriding selected names and omitting selected pages, with behavior covered by the relevant tests.

Written by the indexing model from the issue text.