ADD: update docs for @next version of xchainjs

[mikewyszinski]

Overview

A new version of xchainjs is being undertaken, as part of that effort we should update the docs.

Proposed Changes

1. ADD a cookbook/recipes section - to help newcomers quickly find snippets of common uses for xchainjs
2. ADD an API section - a fully cross-linked JSdoc should be generated and embedded or linked to
3. REMOVE detailed design sections - currently the build step generates documentation based on .md files inside xchainjs , perhaps we should maintain those as documentation for xchainjs maintainers and have this documentation only be for users of the xchainjs lib
4. ADD a sample applications section - @next version of xchainjs will have 2-3 maintained sample applications , the docs should describe them and link to them
5. ADD a getting started section - should have bash and code snippets to quickly start using the library