Infrastructure for api documentation (via Sphinx)
Catch glozz-example up API changes.
Catch up with removal of feature_str
Introduce corpus-wide identifiers for units

Corpus-wide id uses document/subdoc/stage information to help guarantee
uniqueness. Make unit-id private to indicate we'd rather people used
the corpus-wide id.
Exploit STAC convention for subdocument names

Subdocuments start with document name and prefix.
So strip these if we can