docs: Add docs for tracking policy activity

[tianfeng92]

Product Version(s): Calico Enterprise v3.23-1

Issue: https://tigera.atlassian.net/browse/EV-4281

Link to docs preview: https://deploy-preview-2449--calico-docs-preview-next.netlify.app/calico-enterprise/next/observability/elastic/policy-activity

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[matthewdupre]

Couple of small suggestions but I don't need to rereview - thanks

[ctauchen]

Thanks for this, @tianfeng92! I have a few comments in the code. One thing Pasan touched on was the overall structure of the document. I think it might be good to consider something simpler.

[h2] About policy activity logs

• overview of what this is, use case, etc.

[h2] Log structure / Data types

• a table log elements

[h2] Find unused policies

• a step-by-step procedure that explains how to complete this task
• I'm guessing it's 1) run this query 2) get all your policies 3) compare (would be good maybe to explain how to do it?

This represents all the same information have, but presents it in a way that's easier for users to find. Particularly the procedure for finding unused policies.

[pasanw]

Some final comments, no need for me to re-review

[ctauchen]

@tianfeng92 Thanks for the changes, I think that reads well. One nit, then I'm happy to merge and leave for QA.