[86bzbmjk4][accordion] accordion a11y improvements

[ilyabrower]

Motivation and Context

I've changed order of html elements: now it is h3->button instead of button->h3.
Also I've fixed examples:

• removed button in button
• fixed example with custom styles

How has this been tested?

Manually

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Nice improve.

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly or it's not required.
• Unit tests are not broken.
• I have added changelog note to corresponding CHANGELOG.md file with planned publish date.
• I have added new unit tests on added of fixed functionality.

[j-mnizhek]

Looks good overall!

Waiting only for some small changes I've mentioned in the inner discussion.