Offer information about library format in documentation

[mr-smidge]

The exact format of the libraries supported by libdjinterop are currently documented only by the code that implements import/export. It would be better if the format could be documented in plain English and made available via the rest of the documentation.

One option is to have pages generated with Doxygen and added to the existing generated docs on GitHub pages.

An example of the kind of page that would be helpful is on the Mixxx wiki: https://github.com/mixxxdj/mixxx/wiki/Engine%20Library%20format - though this is now out-of-date and only refers to Engine v1.