update header for development

[jo-mueller]

When people start sending PRs to ngff-spec during the ramp-up to 0.6, we should change the header information which currently states:

This is **not** the released version of the ngff-specification. It is an explorative, work-in-progress document.

I suggest to update this to:

This is **not** the released version of the ngff-specification.
It is a work-in-progress document.
Upon release, this warning will be removed and the version number in the document updated.

[github-actions]

Automated Review URLs

• Readthedocs

[joshmoore]

👍

Sidenote:

• zarr-python v2 used commented sections (https://raw.githubusercontent.com/zarr-developers/zarr-python/refs/heads/support/v2/docs/release.rst) to do this
• zarr-python v3 updated to towncrier but not sure if it can provide this type of swap (https://github.com/zarr-developers/zarr-python/blob/acdd892234dbb62833c5d57e59b94d180cd1115d/.readthedocs.yaml#L12)