Rewrite the E911 platform guide to use new native, self-service REST endpoint

[hey-august]

Description

Now that native E911 REST endpoints are live, the platform E911 guide no longer needs to tell people to file a support ticket.

This PR rewrites /docs/platform/e911 around the self-service process:

• create a carrier-validated emergency address
• assign it to a number
• wait for e911_status to become active

The doc uses Fern's built-in endpoint snippet components so the examples render from the OpenAPI spec and can't drift, and it links heavily into the generated E911 Addresses reference.

Also added:

• An Info callout that covers setting EmergencyAddressSid via the Compatibility API
• A brief Dashboard section that covers the E911 tab and per-number assignment.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code cleanup / refactor

Related Issues

#537

Testing

Verified with fern docs md check and fern generate --docs --preview (0 errors); snippets, links, and anchors checked on a preview deploy

• Added/updated unit tests
• Tested manually
• Tested with live SignalWire credentials (if applicable)

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)
• All existing tests pass

[github-actions]

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https://signalwire-preview-docs-538.docs.buildwithfern.com/docs

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