Resolve "Annotation Module: Design of the API Endpoint"

Adds annotations to the TextAPI. We decided to provide them as another key/value pair on collection, manifest and item level in order to decrease surplus endpoint requests: If we had established an obligatory /annotation/endpoint, applications would have expected and queried it – even in cases where no annotation are available. Thus we decided to add an optional field.

Furthermore this MR adds a TOC to the specs page and adds slight corrections to language and style.

Cf. #5 (closed)