New modal for documentation popup

Now uses a Bootstrap modal rather than Thickbox for the modal popup.
This makes us a little more future-proof (WordPress is retiring Thickbox
eventually) and cleans up some overflow issues some uses were
experiencing at certain resolutions.

Also cleaned up the documentation a little, added "back to top" links,
and removed the title and plugin description from the body of the
documentation.

By necessity updated the included version of Bootstrap which styles the
help.