pyndl.corpus is only referenced from API documentation

[frankier]

This issue is part of my JOSS review.

At a high level, I see there being 4 layers/strata in the project from the lowest:

1. The NDL rule as independent of the file format used for the events and the weights matrix (does not seem to be directly exposed/documented -- see the comments of @jinhangjiang and Non-file-based (in-memory/streaming) usage? #213
   • Exposing/documenting this would be a "nice to have" and could create appeal to those outside the cognitive linguistic audience
2. The NDL rule as implemented in terms of the library's custom file formats
   • This is introduced in the documentation through a word form-semantics association (meaning recognition) example
3. Event file generation routines which can convert "END OF DOCUMENT" delimited corpora files into
   • This is introduced in the documentation between a bigram-word form association (form recognition) example
4. Convenience functions to convert from other corpora formats (currently only OpenSubtitles) to stratum 3

Currently, Stratum 4 is only included in the API documentation, and therefore rather easy to miss.