feat(docs): initial sphinx docs build

[chrisdickinson]

The Makefile and make.bat are autogenerated via sphinx-quickstart; generally we'll want to use just docs instead. I left them in in case they're load-bearing for readthedocs (which we'll want to enable when we make this repo public.)

I added some light typing support and, in the process, noticed some rough edges around the @host_fn API – it struck me like there was some work in progress here. In particular the original docstring mentions returning values from the host function, but we don't appear to do anything with them. I also wonder if we should return a full-fledged Function from the wrapper function instead of having to do a two-step:

@extism.host_fn(inputs=[ValType.I32], outputs=[ValType.I64]) # infer name from function name if not given
def hello_world(plugin, inputs, outputs):
   ...

# vs...
@extism.host_fn
def hello_world(plugin, params, results):
    ...
fn = extism.Function("hello_world", [extism.ValType.I64], [], hello_world)

If no one is opposed I'll take a look at taking this on!