Update Zaino page for current Zcash stack - #2011

## Summary

Updates the Zaino page to describe its current role in the Zcash stack.

This PR:
- Explains the roles of Zebra, Zaino, Zallet, and lightwalletd
- Clarifies when standalone Zaino is needed
- Adds current Z3 deployment context
- Adds operator notes and a JSON-RPC exposure warning
- Replaces old future-focused wording with current primary-source guidance

## Verification

- Checked Markdown diff for formatting issues
- Confirmed old wording around zcashd and future functionality was removed
- Cross-checked against the Zaino repo and Z3 deployment docs