fix(email): unsubscribe URL without nested link tags

[Ibochkarev]

Summary

• Nested `[[[[++site_start]]]]` in letter templates becomes `[[[[57]]]]` after `++` expands → MODX logs `Bad link tag`.
• Queue body render adds `[[+unsubscribe_url]]` via `makeUrl` (`sendex_unsubscribe_page` or `site_start`).
• Flattens residual nested resource links before `processElementTags` so existing DB templates keep working.
• Demo template + README updated.

Test plan

• Send a queue mail with a template that still has the old nested tag — no Bad link tag in log; link works
• Prefer `[[+unsubscribe_url]]` in the letter template
• Optional: set `sendex_unsubscribe_page` to the resource with `[[!Sendex]]`