Accessible map content navigation (Pan & zoom + Feature navigation)

[Malvoz]

This is to partly address one of the concerns with this explainer in #17:

and we will enable standardized Web map accessibility through addition of maps to HTML.

How will accessibility be addressed?

Most map libraries today largely or entirely neglect screen reader users and keyboard users.

Accessible map navigation is an area that needs more user research (some can be found in Maps4HTML/chat#13). There has to be a plan to make pan & zoom and feature navigation (map content navigation as a whole) intuitive and accessible. This should probably have a section under Key Scenarios.

In Maps4HTML/MapML.js#396 we're exploring how to make map content navigation accessible in the polyfill <mapml-viewer>, which may inform a standard interaction model for maps.

(The CSSWG proposal for standardizing pan & zoom is relevant, we should reference it.)