[epic] Documentation work

For gulp 4, we are going to be converting the documentation to gitbook and hosting on gulpjs.com.

The list of things that need to be done:

Decide on and finalize a folder structure
Update docs to match completed docs in other modules (vinyl-fs, vinyl, undertaker, etc)
Finish docs in other modules that need completion (bach)
Cleanup recipes, make sure they work with 4.0 and are consistent with each other (superseded by #2164)
Docusaurus setup for the website (ref gulpjs/gulpjs.github.io/issues/31)
Tooling for inlining docs (makes other items easier but not mandatory - ref #1446)