[Docs] Prevent staging deployments from being indexed in searches

### Description

Currently PR staging links for the new EUI+ docs seem to be indexed and showing up in e.g. google searches.
This leads to broken links when the instances are cleaned up.
We already had that happen for the old EUI docs and fixed it in [this PR](https://github.com/elastic/eui/pull/7657) by using `noindex` and `nofollow` for dev & staging.

We should update the new EUI+ docs as well to ensure the PR staging deployments are not indexed.

## Acceptance criteria

* Update Docusaurus configuration to inject a meta tag for PR preview builds.
* Disable sitemap generation in PR preview builds (you can do so through [preset options](https://docusaurus.io/docs/using-plugins#docusauruspreset-classic)).
* Update PR deployment pipeline to set an environment variable that lets docusaurus know it should include the meta tag (see above).