Auto-generate the file-level documentation from its contents

The brief and owner fields in `index.md` for a class and the file that contains it should be inherited (somehow) from the class documentation itself. When there's a single class in a file, it seems silly to copy the brief and owner up to the index file(s).

Another option would be to make those fields `__OPTIONAL__` if there's a single sub-entity in the file. No need to copy if it's just going to be redundant.

Start by tracing how the brief and owner fields are currently represented in class documentation and copied into each file's index.md. Compare the single-class case with files containing multiple sub-entities, then define how inherited or optional fields should behave and what documentation output should confirm the change.

Written by the indexing model from the issue text.