Add deprecation message to the nf-validation docs

[nvnieuwk]

No description provided.

[mirpedrol]

LGTM!

[nvnieuwk]

I'll maybe wait a bit until we coordinate the work better so we can keep the limbo time between the two plugins as short as possible

[ewels]

Why do you need to rename the GitHub actions workflow? I'd prefer to leave that in place, in case we want to add any more notices to the docs in the future.

• Please copy the warning to the docs homepage, in an admonition
• Please add an announcement bar to the docs, visible on every page

We also need to think about how / if to port all issues and PRs over to the nf-schema repository. Once that's done and this PR is merged, we can archive the repo.

[ewels]

This doesn't render properly. The admonition type needs to be on it's own line.

Suggested change

	> [!IMPORTANT] **`nf-validation` has now been deprecated and won't be updated anymore.**
	>
	> **Please update your pipelines to use [`nf-schema`](https://github.com/nextflow-io/nf-schema) instead.**
	>
	> This change was necessary to prevent older versions of `nf-core` pipelines from breaking when updating to the latest version of `nf-validation`. Some older and current versions of the pipelines did not pin the version of nf-validation, which caused the pipelines to break when the latest version of nf-validation was released. This change will prevent this from happening in the future. **Please pin the version of `nf-schema` in your pipeline's `nextflow.config` file.**
	> [!IMPORTANT]
	> **`nf-validation` has now been renamed to `nf-schema`. The `nf-validation` plugin will not receive any future updates.**
	> **Please update your pipelines to use [`nf-schema`](https://github.com/nextflow-io/nf-schema) instead.**
	>
	> This change was necessary to prevent older versions of `nf-core` pipelines from with unpinned plugin references from breaking when updating to the latest version of `nf-validation`.
	> **Please pin the version of `nf-schema` in your pipeline's `nextflow.config` file:**
	> ```nextflow
	> plugins { id '[email protected]' }
	> ```

[ewels]

Should also point to the migration guide.

[nvnieuwk]

Closing in favor of #170