[focusgroup] General clarifications and edits, move previous explainer out of the sidebar

[janewman]

Major Changes in this PR:

• Aligns document around the idea that each focusgroup segment will have a single, guaranteed tab stop, even if the element would normally be focusable via sequential tab navigation, e.g. natively focusable or a tabindex >= 0. This means:
  • Authors don't have to manage tabindex outside of ensuring their items are focusable and managing first-entry order.
  • Users can not tab between focusgroup items in the same segment, even if they have a positive tabindex and would otherwise expect to be in sequential tab order.
• Renames this page to "Focusgroup" and removes the other version of the explainer from the sidebar (still linkable from this document), see: Should "Inactive" be more obvious? #1277 (comment).
• Removes grid examples outside the grid section, and makes the contents of the grid section collapsed. [focusgroup] Should we remove the "future considerations" out of the new explainer? #1295
• Misc. Adds missing or broken links, allows table of contents to be hidden, misc editorial.

[janewman]

@mfreed7 or @gregwhitworth (or any other owner) PTAL and let me know what changes are needed, or please hit "submit" if everything looks good.