Generate jsdocs

[benmccann]

Closes #5821

This PR generates jsdocs that we can link to from the docs to add missing docs as suggested by @jukra

[etimberg]

If we generate JSDocs, should we serve them from somewhere?

[benmccann]

@etimberg Yes, good idea. I added them to the left nav menu (SUMMARY.md)

[kurkle]

I wanted to see how these look like. To build, I changed docsTask to following:

function docsTask(done) {
  var bin = 'gitbook-cli/bin/gitbook.js';
  var cmd = argv.watch ? 'serve' : 'build';

  return run(bin, ['install', './'])
    .then(() => run(bin, [cmd, './', './dist/docs']))
    .then(() => {
      var config = {
        opts: {
          destination: './dist/docs/jsdoc'
        },
        recurse: true
      };
      gulp.src(['./src/**/*.js'], {read: false})
        .pipe(jsdoc(config, done));
    }).catch((err) => {
      done(new Error(err.stdout || err));
    });
}

And here's a example page from the generated docs (printed as pdf to be attachable here):
Documentation Interface_ IPlugin.pdf

A lot of documentation is missing from code to build releaseable documentation.
So I think generation should be limited to the stuff that is readily documented in the code (like this plugin interface).

I think helpers would be a good candidate for documenting and generating next.

[benmccann]

Thanks @kurkle. I've rebased against master using the code you provided

I'm not sure that it's an issue to generate jsdocs for APIs that don't have much documentation yet because I think it may spur contributors to help fill in the missing docs

[kurkle]

Should we go ahead and try this out?

[etimberg]

Am happy to try these out