Revisit Accordion group behavior around forcing one open accordion at a time

[smhigley]

This issue comes from discussion around #1819

We've had comments & observations that the current behavior of forcing one accordion open at a time in the current example is unexpected and confusing, especially as the default behavior. There are some examples where it does make sense to have this sort of managed behavior in a group of accordions, though -- for example, moving through a series of steps where one must be open at a time. Because of that, it does make sense to model the behavior and semantics in some way.

This issue is to discuss how best to do that. Right now we have two suggestions:

1. Add a checkbox for this behavior to the current example, similar to the one on the disclosure widget for navigation example
2. Add a second example with a clearer use case, e.g. a series of steps in a payment process.

[smhigley]

These are two draft PRs that demo each suggested approach:

1. Checkbox in current example: Draft: Accordion example with checkbox to switch behaviors #1833
2. Separate examples: Two accordion examples, one basic and one of a group with only one open accordion #1834