[schema/documentation] Clarify the purpose of links

[odscjen]

From suggestion in GFDRR/rdls-spreadsheet-template#3 (comment) create a codelist for link.rel from https://www.iana.org/assignments/link-relations/link-relations.xhtml. Currently this is just recommended in the field description

[duncandewhurst]

links only exists as a standards-compliant means of declaring the version of the RDLS schema that describes the data (using the 'describedby' code) so I don't think we need to include the IANA codelist in the standard. However, we should update the schema descriptions and documentation to clarify the purpose of links. Sound good?

[odscjen]

That makes a lot of sense to me, @matamadio is that sufficient?

[matamadio]

If the only IANA codelist expected is "describedby", then I don't see the reason to allow the choice to user. The field should have that value in the template already.

[duncandewhurst]

Sounds good. I've created an issue on the template repo and will rename this issue to be about clarifying the purpose of links in the schema and documentation.

[matamadio]

Agreed and implemented (auto-populating with describedby); this can be closed.

[odscjen]

I'm going to leave this open to remind us to include the clarification in the guidance as well as the autopopulation now in the spreadsheet