docs: Add link to RDLS Convertor in navigation menu

[duncandewhurst]

Merge checklist

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
• Update the JSON files in examples

Always:

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

Having trouble?

See how to resolve check failures.

[duncandewhurst]

The link will be broken until #226 is done.