Update banner for master doc versions

[lcawl]

This PR updates the banner that appears in "master" versions of the documentation, since it's currently unclear about the state of the serverless details in that branch.

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[dedemorton]

I wonder if we can change this banner to be less confusing and provide users with a call to action. Maybe something like:

"You are looking at non-versioned documentation that may not be accurate if you're using a versioned Elastic Stack product. To see the documentation for the correct version, use the version selector."

[leemthompo]

Thanks for this @lcawl, I think it makes a lot of sense to tackle some of this confusion in the banner.

I agree with @dedemorton that it would be nice to have a CTA but we also need to address the serverless disambiguation.

Here's a cut, with two blocks of text:

This documentation contains work-in-progress information for future Elastic Stack and Cloud releases. Use the version selector to view supported release docs.

This documentation also contains the latest Elastic Cloud serverless information. For our serverless-specific documentation, check out our serverless docs.

[lcawl]

Here's a cut, with two blocks of text:

I like that suggestion @leemthompo but I pruned it down a bit since the two blocks of text took up a lot of room at the top of each page.