Add wallets guide

[s-ben]

Looking to do a wallets guide, similar to the Bitcoin dev docs wallet guide. Basic idea is to have a few high-level pages explaining basic concepts (perhaps filtering Bitcoin pages as a first pass), some pages that provide the technical documentation a wallet developer (or integrator) will need. My first inclination is to pull all of the (high quality) technical docs from the /dcrd and /dcrwallet repos into dcrdevdocs. It would be nice to have it all in one place, would look prettier and also make the v1.0 release more substantial. However, it might make more sense to keep some of that documentation "decentralized" in repos where it's easier for contributors to update. There could also be cases where a user will expect it there (e.g. installation instructions? repo-specific guidelines?). Therefore, I thought I'd just list the docs I think could be good for devdocs, see what people thought we should subtract/add. Also open to any ideas around wallet docs generally. Especially things we get asked in #support.

dcrwallet

• README
  • RPC API Specification
  • Client usage
  • Making API Changes

dcrd

• README
  • JSON-RPC Reference
  • Go Modules
  • Module Hierarchy

[jholdstock]

Wallet guide was started in #50