Schema tidy-up tasks before Alpha release

[duncandewhurst]

• Add links to codelist reference documentation in schema descriptions. Fields that refer to a codelist should include a link to the reference documentation for the codelist.
• Run OCDSKit's schema-strict. I think that we want all of the validation keywords added by https://ocdskit.readthedocs.io/en/latest/cli/schema.html#schema-strict.
• Add references to details fields in schema descriptions. The schema description for fields that are paired with a details field should include a reference to the details field.
• Update canonical schema link #62