Document btwallet

[MichaelTrestman]

the btwallet library has no proper docs :(

I only realized this when I went to look up the answer to a question in discord "how do i create a wallet from mnemonic in btwallet".

Our docs should cover this, but more generally we need to generate docs for the python wallet SDK the way we do for the core bittensor sdk python library, with sphinx or another framework. This just slipped through the cracks historically, and is a pretty big oversight.

https://github.com/opentensor/btwallet

@chideraao pair with @MichaelTrestman on this so you can learn the docs gen process