webid serializations consensus 2023

https://github.com/solid/solid-wg-charter/issues/39

The consensus for WebID serializations ~10 years ago was:

A WebID Profile Document is a Web resource that MUST be available as
text/turtle [turtle], but MAY be available in other RDF serialization
formats (e.g. [RDFA-CORE]) if requested through content negotiation.

Since then there have been about a decade of discussions.  We should
probably make the views of the CG known before it is handed off to the WG.

JSON-LD has matured and gained significant adoption in the intervening time
period, and IMHO, should be a primary serialization.

A starting point IMHO reflecting consensus could be:

A WebID Profile Document is a Web resource that MUST be available as
text/turtle [turtle], MUST be available as application/l+json [json-ld],
and MAY available in other RDF serialization formats if requested through
content negotiation

ie
- Turtle MUST (+1)
- JSON-LD MUST (+1)
- others MAY

I am unsure if we should vote and tally them, or preferably avoid voting
all together and agree unanimously.  If anyone disagrees, they should be
heard tho.

Minor details:
- JSON-LD can also be delivered in a structured data island
- I removed the reference to RDFa, as I Think it's superciced, some may be
agaisnt that, but I think we can establish group consensus there

I think handing that off to the WG would be a win all round.  Worth adding
your view, as it might be the last time we get a say on WebID in this group.