docs(button): custom roles

[bennypowers]

Closes #2805

What I did

1. document how to apply custom roles to <pf-button>

Testing Instructions

1. use browser a11y tools to audit the dp demo

Notes to reviewers

Don't merge this into chore/lint-ts-55. Instead, after that merges, change the target of this pr to staging/4.0 and merge into there.

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[kelsS]

@bennypowers this looks good to me and makes sense, however I only want to use the custom role for the carousel slider buttons temporarily. Ultimately I would like to help get a carousel slider component itself into the design system. I posted more comments and asked for your thoughts in our Slack thread. I look forward to hearing from you!