docs(inbound-email): add greenfield framework pages (.NET, Spring, Go, Phoenix, Symfony)

[mpge]

Summary

Adds inbound-email setup pages for the five greenfield framework ports that came online in the iter 71–88 rollout. Each page documents:

• Unified webhook endpoint — the single POST /{prefix}/webhook/email/inbound route with ?adapter=postmark|mailgun selection (or the X-Escalated-Adapter header).
• Shared-secret configuration — symmetric secret used both to verify inbound webhooks (timing-safe comparison) and to sign outbound Reply-To addresses.
• Provider setup — exact Postmark + Mailgun webhook URLs + the X-Escalated-Inbound-Secret header to attach.
• Test recipe — ready-to-paste curl command.
• Response shape — includes the new outcome, ticket_id, reply_id, and pending_attachment_downloads fields that the orchestration service returns.

Also rewrites _intro.md to reflect:

• Current supported-providers set: Postmark + Mailgun out of the box, SES and IMAP are no longer advertised as built-in (they'd require custom parsers).
• Dual-mode webhook architecture: unified endpoint on greenfield frameworks; provider-specific endpoints on the legacy host-app integrations (Laravel, Rails, Django, Adonis, Filament, WordPress).
• The Message-ID / signed Reply-To / subject-reference resolution chain that replaces the old subject-only matching.

Files

• sections/inbound-email/dotnet.md (new)
• sections/inbound-email/spring.md (new)
• sections/inbound-email/go.md (new)
• sections/inbound-email/phoenix.md (new)
• sections/inbound-email/symfony.md (new)
• sections/inbound-email/_intro.md (rewritten)

Test plan

• Preview the tabbed section in the docs builder — verify all 11 framework tabs (legacy 6 + greenfield 5) render.
• Cross-check each curl recipe against the actual running service (manual smoke test).