Add extensions to `docgen` and make it gitbook compatible

[chmanie]

Summary

Currently the extensions are not documented in the auto-generated markdown docs using the docgen script. We need to add those. Furthermore the docs currently rely on an old proprietary format that was used for the former Colony developer platform (that doesn't exist anymore). We are moving to GitBook now. For that we should try to generate a similar output to the TypeScript project documentations that already exist.