[WIP] [DOCS] Adds General Purpose LLM v2 connector docs

[szabosteve]

Summary

This PR makes updates on the Elastic Managed LLM page:

• renames the page to Elastic Managed LLMs
• adds a new section about General Purpose LLM v2
• renames the section about Elastic Managed LLM to Elastic Managed LLM v2

TO DO

• Update the model card for v2

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Documentation was added for features that require explanation or tutorials

[elasticmachine]

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[seanhandley]

I think it makes sense to refer to the original LLM as "General Purpose LLM v1".

They are both considered "Elastic Managed LLMs"

[szabosteve]

@seanhandley Okay! Does that mean that Elastic Managed LLM will be renamed to General Purpose LLM v1 on the UI? I wanted to keep the original terminology in the docs for parity with the UI.

[seanhandley]

Yes #243094