Update index.md separate reference and guidance

[stufraser1]

Updates to make clear RDLS is for metadata.
Includes text change and adding separation of guidance from reference in the bullet list. No change made to TOC

Related issues

Partially addresses #160 - decision made in tooling session to keep public guidance but more clearly separate reference section from guidance.

Merge checklist

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

If you added or removed a field:

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

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[stufraser1]

We discussed a change in page structure do we include separation of the overview (containing core standards, use cases, history) to make at least use cases more prominent in nav bar?

[duncandewhurst]

I think the landing page content needs to be looked at in conjunction with the overall structure/navigation of the docs. Building on our discussion and your changes in this PR, I've proposed a revised structure and landing page content in #43 (comment)

Edit: Once agreed in the issue, we can update this PR with the changes.

[duncandewhurst]

Closing as the changes were made in #175