docs: clarify historical fetch order book usage

[realfishsam]

Summary

• Clarify that Polymarket fetch_order_book(token_id) without historical params uses the live CLOB only and can fail for closed/resolved markets.
• Document archive-backed historical snapshot/range behavior and distinguish it from live order book calls.
• Use fetch_event(slug=...) with nested markets for crypto 5m event slugs.
• Fix curl Authorization header quote syntax and note raw bulk Parquet access via PMXT Archive.

Closes #836

Test plan

• Inspected package.json docs scripts; no dedicated docs/MDX validation script was present.
• Ran a Node sanity check for balanced MDX code fences and component tags, plus curl Authorization header syntax.
• Ran git diff --check.

[mintlify]

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