Support variables in frontmatter description

[eedugon]

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 (done in Remove outdated hardcoded version from frontmatter docs-content#3545)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.

[bmorelli25]

This should absolutely be programmatically updated. I'm tempted to just add this to the @elastic/docs-engineering board and assign it as an eng tasks to fix however that team sees fit (using {{version.stack}} or otherwise).

[florent-leborgne]

Found this while looking for cumulative docs related issues. While not related, I found that this hardcoded value is now surprisingly outdated :D, so I'm removing it in elastic/docs-content#3545

[florent-leborgne]

We should also transfer this issue to docs-builder