Add CrateDB CRD reference page

[tomach]

Summary of changes

Adds a new documentation page, CrateDB CRD reference (docs/source/crd-reference.rst), and links it into the toctree in index.rst.

The operator's cratedbs.cloud.crate.io CRD had no human-readable reference. Since we recommend the operator for self-hosted deployments, this fills that gap with a single page describing the important settings and where they go.

Checklist

• Link to issue this PR refers to: https://github.com/crate/cloud/issues/3104
• Relevant changes are reflected in CHANGES.rst
• Added or changed code is covered by tests
• Documentation has been updated if necessary
• Changed code does not contain any breaking changes (or this is a major version change)

[plaharanne]

💯