Subannotation documentation

[mmcauliffe]

There needs to be more explanation of subannotation enrichment (like the functionality developed for AutoVOT, as well as more generally). The name "subannotation" is still not great, but since annotation is used to refer to linguistic annotations like words/phones/etc, I'm not sure what a better name would be. Meta-annotation seems clunky. Regardless, some explanation of what subannotations are and how they differ from annotations is needed at some place.

User documentation:

• Add page for "Subannotation enrichment" to https://polyglotdb.readthedocs.io/en/latest/enrichment.html (might be a subset of Enrichment via CSV files)
• Add page for querying/exporting subannotations to https://polyglotdb.readthedocs.io/en/latest/queries.html

API documentation:

• Docstrings for subannotation import functions

(There isn't currently a place for the subannotation from CSV functions to be in the API documentation, this will be part of #137)