Description

This PR updates the accordion component to update its buttons' and corresponding content elements' aria-expanded and aria-hidden attributes (respectively) when initialized. This means that we can strip all of the aria-expanded="false" and aria-hidden="true" attributes from the HTML, which makes the content accessible in the event that the JS isn't loaded (or until it is). Hopefully, this addresses #1122.

⚠️ Note that the biggest change here is not in the JS, but in the HTML: Users will be expected to update their HTML to remove aria-expanded="false" and aria-hidden="true" attributes from accordion panels that aren't expanded by default so that their content is accessible without JS.

Additional information

• The accordion documentation (and all of the accordions in each component's documentation) markup now reflects the best practice of leaving aria-expanded="false" and aria-hidden="true" attributes out, so that the content is accessible without JavaScript.
• I've updated the accordion documentation to state that the aria-expanded and aria-hidden attributes are unnecessary now (except in the case of the content area that you wish to be expanded by default), but this could probably use some clarification and/or word-smithing from @kategarklavs.
• The demo HTML now uses the amendment-n+1 naming convention instead of collapsible-n, so that it plays nicely with the the Code and Documentation accordion below. See: [UI components] Documentation pages have duplicate ids #1225
• I've added jsdoc comments to all of the Accordion methods, and updated some of the existing ones.
• The Accordion JS component also now returns this from its show(), hide(), and hideAll() methods, so its methods are chainable. I'm happy to back out of this change if it's not consistent with the rest of the library.