feat(design): clean up accordion component and update docs

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Accordion docs are not very well written. Accordion styles need to be updated to be more aligned with other Daffodil components.

Part of: #2767

What is the new behavior?

1. Deprecate initiallyActive property and change it to initiallyExpanded.
2. Deprecate daffAccordionItemContent. It's not necessary to have this directive.
3. Clean up styles
4. Update header to use <button> element
5. Add accessibility considerations
6. Write better docs

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information