Small fixes for API keys docs

[kilfoyle]

This updates the Grant standalone Elastic Agents access to Elasticsearch page:

• Indicate that API key authentication is required for serverless.
• Correct the API key format description (delimiter is : rather than .).
• Provide the Kibana path for serverless, since it's different from the on-prem and ESS versions (I confirmed that the path is Project settings -> Management -> API keys in the Obs, Sec, and Elasticsearch projects).

Apart from the above, the docs seem to be nicely up to date.

Closes: #697

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[amyjtechwriter]

LGTM

[kilfoyle]

@elasticmachine run docs-build

[kilfoyle]

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Status	Branch	Result
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