docs: document standalone mode runners

[Hashim1999164]

Summary

• Documents the top-level standalone mode runners (navigation, startTimespan, snapshot) in docs/user-flows.md.
• Clarifies how they differ from flow.navigate / flow.startTimespan / flow.snapshot, including that standalone runs do not attach to startFlow reports.
• Adds a short link from docs/readme.md so programmatic users can find the guide.

Fixes #15891

Test plan

• Confirm API names match exports in core/index.js
• Skim the new Standalone mode runners section for accuracy and tone
• Open the markdown locally and check formatting of tables and code blocks

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[Hashim1999164]

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