Document how to disable screen reader functionality using speak_text variable

[Copilot]

This PR adds comprehensive documentation for the speak_text variable that allows developers to disable the Assembly Line's built-in screen reader functionality on a per-interview or organization-wide basis.

Changes Made

Added a new "Screen reader control" section to the magic variables documentation (docs/components/AssemblyLine/magic_variables.md) that includes:

• Purpose explanation: The screen reader was added for people with low literacy and users on shared workstations or borrowed mobile devices, not primarily for blind users
• Configuration options: How to set speak_text = False to disable the functionality
• Per-interview examples: Code snippets showing how to disable in individual interviews
• Organization-wide examples: How to configure globally via shared YAML files
• Reference link: Points to official Docassemble documentation for additional context

Example Usage

Per-interview configuration:

---
mandatory: True
code: |
  speak_text = False
  # ... rest of your interview order
---

Organization-wide configuration:

---
code: |
  speak_text = False
---

The new documentation section is properly integrated with the existing table of contents and follows the same formatting patterns as other variable documentation.

Fixes #192.

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[nonprofittechy]

@BryceStevenWilley yep, confirming this was my mistake in the PR review