More API docs.

[ctb]

Questions like those in #289, #305, and #308 should be answered in the docs!

[ctb]

Also include examples like this: https://gist.github.com/ctb/40da5a63a173dfdf4b109ffde7b76874#file-build-and-search-py

[ctb]

e.g. see this file, which may no longer run the tests, now that it's .md...

things like getting hashes from signatures, signature merge, constructing new signatures and adding hashes, downsampling, etc. could be done here - could do in a jup notebook / binder, too.

[ctb]

The build-and-search example is now in the docs with a full Python example - see #436.