Add API docs

[jgaskins]

Use GitHub pages for this and generate the docs with this command:

crystal docs --project-version=$VERSION --source-refname=v$VERSION

A script should probably be written to run that that, which would require a version to be supplied. Ideally, it would work the way the Crystal folks support multiple versions in their API docs with a dropdown (see image below), but I don't know how they do that. The retroactive documentation can be added later, though.

I'll have to enable GHP on this repo, as well, which will host it at https://jgaskins.dev/neo4j.cr