Fix up the intro docs

[ethanfrey]

• (docs: clean up basecoin-basics #214) Check out if the basic setup makes sense:
  https://github.com/tendermint/basecoin/blob/unstable/docs/guide/basecoin-basics.md

• (docs: key management guide #216) Explain key management:
  https://github.com/tendermint/basecoin/blob/unstable/docs/guide/key-management.md
  (you can just use basecli keys --help to see what is there and play around, especially with the recovery seed).

[zramsay]

First part done in #214. The guide is a great getting start tutorial & does make sense. It would be a tad easier with #213 and we then link to the key management file for advanced keys.

Second part done in #216.

[ethanfrey]

great. thank you!