Attached detached lite

[ptsefton]

This is another approach to clarifying Attached vs Detached that does not introduce much new terminology or conformsTo etc. Again, this is a first pass to see if this approach makes sense, it will need checking.

I moved a bit of stuff around in structure.md but there are no major changes there except to cover how to deal with a Detached RO-Crate Package. Also, did not try to deal with how you'd link one to a website.

In the section on Data Entites i further tidied up the logic around @ids and contentUrls -- I think this has made it clearer, and I don't think it will be hard to implement.

Thanks for your feedback @simleo about the complexity in my last try.

[elichad]

Thanks for the updates @ptsefton . I found a couple more comments, and there are still some of my previous comments waiting to be addressed.

[ptsefton]

Quick comment with my understanding of the primary spec changes in this PR (i.e. ignoring restructuring & rephrasing):

Previously

This table represents (my understanding of) the difference between attached/detached crates prior to this PR:
Data Entity @ids all absolute URIs Data Entity @ids include local URIs
RDE has @id ./ Attached Attached
RDE has @id absolute URI Detached Not permitted

The RDE @id was a major factor (by proxy) in defining whether a crate was attached or detached.

Now (27 Jan)

Data Entity @ids all absolute URIs Data Entity @ids include local URIs
RDE has @id ./ Attached Attached
RDE has @id absolute URI Attached or Detached Attached

The RDE @id can no longer be used as a proxy for whether a crate is attached or detached. Instead this PR makes clearer the following distinction that it's about how a crate is being distributed or processed:

* an Attached crate package is a _directory containing the metadata file_

* a Detached crate package is a _standalone metadata document_

I'll add specific review comments tomorrow (out of time today) but @ptsefton does this sound right to you?

Now (updated 6 Feb)

Some additional edits were made, so the table now looks like this:
Data Entity @ids all absolute URIs Data Entity @ids include local URIs
RDE has @id ./ Attached or Detached Attached
RDE has @id absolute URI Attached or Detached Attached

I think the final summary is correct but kind of backwards. It should not be up to validation software or libraries to divine the type of a crate from what it sees, rather you would process it in Attached or Detached mode depending on the context and report any errors. I think this is clear from the section on Attached and Detached RO-Crate Packages - are you calling it with a path to a directory or to a file/API?

[elichad]

I'm happy with this now. I'll make a couple of commits to fix the CI and some typos I spotted on my final look through, then merge

All comments have been addressed