documentation site

[lukavdplas]

Adds a documentation site based on MKDocs. At the moment, you need to clone and use mkdocs serve to host it locally, but I'll bring it online after merging. (close #2)

Because the documentation is largely generated from the code, I also improved the clarity there:

• Substantial expansion docstrings. Every function that is documented in the API reference now has a docstring that lists arguments and what the function returns.
• Docstrings use standardised formatting (google-style) for parameters and the returned value
• Added type annotations to most API functions
• Adds a unit test for each reader type. This was mainly to make sure my type annotations did not contain syntax errors. (close add unit tests #1)

Also:

• updates some error messages that referenced "CorpusDefinition"
• updated some argument names in csv-related functions that take a column name, to say column rather than field. This makes argument hints easier to understand, but I've kept class members as-is, so it does introduce some inconsistency.
• removed some stray functions in readers.core that were just left over from I-analyzer.

Request for review: the documentation is fairly straightforward, so don't worry about checking every detail. I'd be happy if you can check if the above list of changes sounds good and glance through the documentation site.