Don't require accordion sections to be siblings.

This will allow accordions that have their sections separated into
different block-grid areas.
Improve documentation.
Don't require accordion sections to be siblings.

This will allow accordions that have their sections separated into
different block-grid areas.
Improve documentation.
Merge remote-tracking branch 'github/accordion-split-sections'
Allow accordion event to trigger on embedded accordion sections.

Fixes issue #4420.
Add test for block-grid accordion.
Merge pull request #4421 from tejohnso/accordion-split-sections

Accordion split sections