Remove custom subpaths callout

[ethanpalm]

Documentation changes

Custom subpaths aren't available out of the box, so this PR removes information implying that they are.

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For Reviewers

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✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
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✅ User experience

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Note

Docs update: /deploy/docs-subpath.mdx

• Removes the "Custom subpaths" section implying arbitrary subpaths are supported
• Rewrites the strict security policies note to clarify configuring proxy/firewall rules to add correct headers

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