[FEATURE] Support for accessibility tags

[jbednar]

Problem description

Bokeh makes it easy to build web apps, but as far as I can see, there isn't an easy way to make them accessible for screen readers or other non-graphical interaction or inspection.

Feature description

It would be great if some low-level base class in Bokeh would support some of the ARIA tags so that an app author could easily supply human-readable text describing what a given plot contains or what a given layout contains or what a given control does.

Potential alternatives

There's a bit of overlap with the recent tooltips work, but I think this is really a separate request.

Additional information

No response