docs: add Mermaid diagrams to README and docs

[christiangda]

Summary

Adds GitHub-rendered Mermaid diagrams across the README and docs so the architecture, message flow, lifecycle, and SMTP TLS handshake are visual. Replaces the previous ASCII-art diagrams. Docs-only — no code changed.

What's included

README.md

• Architecture flowchart — producers → builder → buffered queue → worker pool → MailerService → SMTP/custom backend.
• Message-flow sequence diagram — Start → Enqueue (with the back-pressure branch) → worker → Send (dial/TLS/AUTH/DATA) → Stop drain.
• Service-lifecycle state diagram — Ready → Running → Draining/Cancelled → Stopped.

docs/production-guide.md

• Layered-architecture flowchart (construction / queue+workers / transport).
• Shutdown state diagram.
• Enqueue back-pressure decision flowchart (stopped? / queue full? / ctx cancelled?).
• SMTP TLS/STARTTLS negotiation flowchart (implicit TLS vs STARTTLS vs RequireTLS failure).

docs/examples/basic.md

• Five-step flow diagram.

docs/examples/custom-backend.md

• MailerService class diagram (interface, MailContent accessors, implementations, RetryMailer wrapper).
• Added a concrete RetryMailer wrapper example the diagram references.

Notes

All diagrams use standard Mermaid, which GitHub renders natively in Markdown. State-transition labels avoid <br/> (rendered literally there); node labels use it for wrapping.

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