Add guidance that all URIs should be absolute, and not relative

[ThisIsMissEm]

I picked this up from @NSeydoux on the Inrupt team, where the OAuth spec doesn't actually require absolute URIs for various properties like logo_uri, tos_uri, etc.

Ref: solid/solid-oidc#231

[Zegnat]

I was trying to figure out if there was any earlier discussion about this for RFC 8414, Authorization server Metadata. But I was not able to find anything. Feels like that would have had the same issue, but it has never come up?