docs: add guide for email address reading in voice agents (#1024)

* docs: add guide for AI voice agent email address reading

Adds a comprehensive guide covering how to configure voice agents to
collect, read back, and confirm email addresses clearly. Includes
copy-pasteable system prompt examples, references to Vapi's built-in
formatEmails voice formatter, and best practices for the full
collection-and-confirmation conversation flow.

Resolves DEVREL-621.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add API configuration section for email formatting

Expand the email address reading guide with a technical API
configuration section covering:
- Full 14-step voice formatting pipeline table showing where
  formatEmails (step 8) fits
- FormatPlan TypeScript type definition
- Code examples (JSON, TypeScript SDK, Python SDK) for enabling
  specific formatters, disabling email formatting selectively,
  and disabling all formatting
- Note on formattersEnabled introduction date (2025-02-20)
- Updated overview to describe three solution layers (built-in
  formatting, API configuration, prompt engineering)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: adhamvapi <256238690+adhamvapi@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: adhamvapi