Add Perl SDK reference documentation

[fern-api]

Summary

Adds complete Perl SDK reference documentation (~678 pages) by transforming the existing Python SDK reference pages into Perl equivalents.

Changes

• 678 new Perl reference pages under pages/reference/perl/ covering:
  • Agents namespace (335 pages): AgentBase, SWMLService, SWAIG, skills, contexts, etc.
  • RELAY namespace (91 pages): Call control, messaging, events, etc.
  • REST Client namespace (252 pages): Fabric, calling, phone numbers, etc.
• Perl variant registered in sdks.yml with camel icon
• Core overview made language-neutral (shared across all SDK variants)
• 49 guide pages updated with uncommented Perl-specific content (43 Tab blocks, 88 table rows)

Transformation Details

• All Python types converted to Perl equivalents (str → $scalar, list[T] → @array, etc.)
• All code examples rewritten in Perl with proper syntax (-> chaining, => fat comma, my $ declarations)
• All internal links updated from /python/ to /perl/ paths
• 122 Python-only pages appropriately skipped (no Perl SDK equivalent)

Verification

• Zero Python slugs remaining in Perl pages
• Zero Python links remaining in Perl pages
• Zero Python code blocks remaining in Perl pages
• Zero Python type syntax remaining in Perl pages
• All guide page Perl tabs properly uncommented and closed

Requested by: Fern Support

[github-actions]

❌ Link Check Failed

Preview: https://signalwire-preview-e8f07273-6aef-44cf-96df-727cc7bf3d2a.docs.buildwithfern.com/docs

❌ 5 Broken Link(s)

Broken URL	Status	Found On
https://server.com/sales	404	https://signalwire-preview-e8f07273-6aef-44cf-96df-727cc7bf3d2a.docs.buildwithfern.com/docs/sdks/guides/mapping-numbers
https://server.com/billing	404	https://signalwire-preview-e8f07273-6aef-44cf-96df-727cc7bf3d2a.docs.buildwithfern.com/docs/sdks/guides/mapping-numbers
https://server.com/support	404	https://signalwire-preview-e8f07273-6aef-44cf-96df-727cc7bf3d2a.docs.buildwithfern.com/docs/sdks/guides/mapping-numbers
https://github.githubassets.com/	404	https://github.com/signalwire/docs/issues/new/choose
https://your-server.com/agent	Network error: Network unreachable. Check internet connection or VPN settings (error sending request for url (https://your-server.com/agent))	https://signalwire-preview-e8f07273-6aef-44cf-96df-727cc7bf3d2a.docs.buildwithfern.com/docs/sdks/guides/phone-numbers

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[fern-api]

Requested by: Devon White
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