a11y metadata proposal

[HadrienGardeur]

This draft PR contains an initial proposal for handling a11y metadata in RWPM. It only contains a JSON Schema for now.

~~Among the things that are still missing in the JSON schema:~~

• ~~no enum for feature, we could extract https://www.w3.org/2021/a11y-discov-vocab/latest/#accessibilityFeature~~
• ~~no enum for hazard, we could extract https://www.w3.org/2021/a11y-discov-vocab/latest/#accessibilityHazard~~

Here's also a checklist of things that need to be handled before this becomes a real PR:

• [x] add these properties to our JSON-LD mapping document
• [ ] add these properties to our default metadata profile
• [x] and of course, discuss this with the group and get a broad agreement on the approach and naming conventions