base: cockpit.js can be loaded without jQuery

This is necessary for the AMD module loader to work which will in
turn locate and load jQuery in later commits.

In addition this gives developers using things like dojo in packages
a small API for interacting with cockpit-ws and the message stream.
doc: More spacing between entries in guide reference sections
examples: Add some examples of building a simple component

This currently uses the #playground task since we don't have a way
to add arbitrary tasks to the Cockpit UI.

Closes #1399