Add a new section detailing the various potential realms

This adds a new section under scripts detailing the entry, incumbent,
current, and relevant concepts with regard to realms/global objects/
environment settings objects. This centralizes information that was
previously somewhat spread out, and adds lots of details, as well as
an example.

This takes care of some of the pain points noted in #473, in particular
fixing the definition of entry settings object to be more clearly
defined in terms of the JavaScript execution context stack. There is
still some work to do in that issue: namely, updating Web IDL with
regard to tracking the entry settings object, and also making sure that
incumbent settings object is correctly defined.

This closes #167 by finally implementing the plan in
#167 (comment) for
making the correspondences clear.