add basic documentation

[rafaqz]

Set up documentation, and tweak the current docs a little.

[rafaqz]

Thanks for the edits. The readme also doubles as the module docs now.

[rafaqz]

Are you happy to register v0.6 when this is merged?

[visr]

Yeah let's register it. Not sure how to write an upgrade guide much better than "everything changed, look at the new docs". The README still needs a link to the docs.

[visr]

Neat trick to add the readme as module docs by the way, it looks much better now: https://juliageo.org/geojson.jl/dev/.