Document kuery (KQL) filtering and improve Fleet API page structure

[charlotte-hoblik]

Summary

Adds KQL filtering guidance to the Fleet API page and brings the page in line with the how-to content type. Closes #3401.

Connected issue: #3401

kuery / KQL filtering (the issue)

• New Filter results with KQL section: which list endpoints accept kuery, the saved-object field-prefix rule, a verified package_policies example, and links to the KQL reference. The API reference names kuery but never explains it or links out, this is the missing on-ramp.

Structure and navigation (applied the Elastic docs skills)

• Rewrote the page opening to state purpose and scope, and added a description to the frontmatter (no longer duplicated by the intro).
• Added a Before you begin prerequisites section, plus Next steps and Related pages sections.
• Folded the standalone Authentication section into the prerequisites.

Copy edits throughout

• Active voice, concision, and punctuation; removed the relative-time word "currently"; replaced a directional "previous example" reference with a direct link; fixed an unclosed quote in a curl example.

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used:
Claude Opus 4.8
Elastic docs review skills

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🔍 Preview links for changed docs

• reference/fleet/fleet-api-docs.md

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