Fix broken codelist reference URLs in schema

[duncandewhurst]

Related issues

• [Docs update] Fix codelists horizontal & vertical scroll, check hyperlinks #161

Description

This PR replaces the _ character with - in codelist URLs. It also adds a style guide for codelist field descriptions so that is done on an ongoing basis.

Merge checklist

• Update the changelog (style guide)
• Run ./manage.py pre-commit

If you added, removed or renamed a field:

• Update the collapse option of the jsonschema directives for dataset, resource, hazard, exposure, vulnerability and loss on reference/schema.md
• Update the diagrams in reference/schema/md

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