add external references to the ro-crate for DMP and SOP

[marc-portier]

We should reference in a formal way:

• SOPs
• DMP

and include them in the metadata

[kmexter]

This is more for @laurianvm and @kmexter to do, as we are supposed to create an ideal ro-crate and extra-metadata.
I cannot find an issue on this (impossible to find random issues in GH!) so I will adopt this one

Create a good ro-crate for each observatory: we have done a good job so far, but there is extra metadata that needs to get in AND needs to display in the ro-crate viewer well (yes, two separate issues), and we need to decide on the metadata to go into an "extra metadata" file to be incorporated in the ro-crate (this file being a fixed, not deletable, part of each observatory repo).

See confluence notes on https://vliz.atlassian.net/wiki/spaces/VMDCOS/pages/93783049/EMO+BON+RO-Crates.

[cpavloud]

This is relevant to emo-bon/sequencing-crate#16 and #10