Support variables in frontmatter description

The current release docs landing page has this in the frontmatter:

```
description: Explore the new features, enhancements, fixes, and deprecations for Elastic Stack 9.0+ (latest 9.1.1), Elastic Cloud Serverless, and...
```

File: `release-notes/intro/index.md`.

The static `9.1.1` will force us to update it after every release, which is something we want to avoid at all cost (because we will end up forgetting about it).

We should rely on {{version.stack}} but variables are not supported in the frontmatter.

Possibilities:
- Remove the `(latest 9.1.1)` text from the description. (done in https://github.com/elastic/docs-content/pull/3545)
- Use a version variable and support it at docs-builder level. (still valid to consider - renamed the issue accordingly)

Based on Jan's comment, the description in the frontmatter is added as the meta description of the htlm head (SEO related).

cc: @georgewallace & @bmorelli25 , for your consideration.