[Serverless] Adds entity store docs

[natasha-moore-elastic]

Contributes to #5836 by adding serverless documentation for the new entity store feature.

These docs we already reviewed when they were added to ESS in #6053. The only difference is the content on the Entity risk scoring requirements page.

Previews:

• View risk score data | Entity Analytics dashboard
• Asset criticality | View and assign asset criticality
• Entity store
• Entity risk scoring requirements
• Entity Analytics

[github-actions]

A documentation preview will be available soon.

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