Run accessibility audits

[trallard]

Follows #8 - once the plan is completely solidified, I will add the scope here.

Expected deliverables

• Audit assets and findings are public at https://github.com/Quansight-Labs/bokeh-a11y-audit (final format to be tweaked, but we initially agreed on md files for content)
• This should serve as the starting point for the accessibility roadmap

[frankelavsky]

I've got a template of the first issue ready! Would love to have write access to the bokeh-a11y-audit repo so I can get a branch + PR + and discussion rolling! Thanks so much.

[trallard]

Sorted @frankelavsky!