General overhaul / cleanup of docs

[ian-h-chamberlain]

Probably blocked by #42 since the external-facing API is still in a lot of flux. But in general the cargo-generated documentation should be at a place where multiple types of clients could use it (and in theory a new developer could come in and write a new app against the library).

Also related to #17 – basic documentation should be published to the site via github pages if possible.

Finally, this issue should also be used to add/update a proper CHANGELOG.md. The one in the vscode-redcode repo is probably a good example.