Focusgroup updates (#1021)

First revision relative to addressing issues #537 and #989:

## Sections updated (and attempts at simplification):
   * introduction.
   * goals
   * non-goals
   * principles
   * use cases: drop extend, drop multiple focusgroups on one node.
   * focusgroup concepts
   * wrapping
   * axis-limiting
   * opt-out
   * grid updates

## Sections removed:
   * 'extend' concept and orthogonal focusgroups
   * multiple focusgroups for siblings

## Sections added:
   * disabling focusgroup memory
   * sequential focus navigation algorithm updates
   * feature detection
   * design decisions

## General document hygiene:
   * Adding a quickstart section
   * Remove numbers from headers

***Make this an Active explainer again.***