Ad-hoc terms can be moved to a Profile Crate

[stain]

Links the Ad-hoc section to the new profile section and suggest using that profile as a @context to avoid repeating terms.

[stain]

Rendered at:

• https://stain.github.io/ro-crate/1.2-DRAFT/appendix/jsonld#grouping-extensions-as-an-ro-crate-profile
• https://stain.github.io/ro-crate/1.2-DRAFT/profiles#extension-vocabularies
• https://stain.github.io/ro-crate/1.2-DRAFT/profiles#json-ld-context

[stain]

Merging as call 2023-03-23 agreed to merge all outstanding PR (that are not WIP) so we can do an overall review and push out 1.2.