chore(core): Generate docs for http and cookies plugins

[jcesarmobile]

As far as I know the http and cookies docs on https://github.com/ionic-team/capacitor-docs were once manually generated and are not up to date because of that.

This PR adds a dev dependency to @capacitor/docgen in core and adds a docgen script to fill the API data in the new cookies.md and http.md files.