Break unwiedly api doc into multiple files
The api doc is now very large and should probably be broken up into a separate file per module.
Right now the wallet and consensus are the only ones that are implemented, so this is a bit more of a long term issue.
I'm open to other ideas for breaking up the api doc. Perhaps we could put it on learn.siacoin.com instead of github? Then we can use javascript to make it fancier.