Add documentation for Purism Boot Modes (Basic and Restricted)

[Copilot]

This PR adds comprehensive documentation for the Basic and Restricted boot modes that Purism has developed and upstreamed to the Heads project. These modes provide different security/usability trade-offs for users with varying requirements.

Changes Made

New Documentation Page:

• Created /Installing-and-Configuring/PurismBootModes.md that explains both boot modes
• Provides clear descriptions of Basic mode (simplified security) and Restricted mode (advanced security)
• Links to authoritative Purism documentation for detailed implementation information
• Includes guidance on choosing between modes based on user requirements

Cross-References Added:

• Updated Prerequisites.md to reference the new documentation for users who prefer not to use USB security dongles
• Updated the Installing-and-Configuring index page to include boot modes in the security section

Documentation Approach

Following documentation best practices, this implementation:

• Provides brief, clear explanations of what each mode offers
• Links to Purism's authoritative documentation rather than duplicating detailed technical content
• Explains how these modes fit into the broader Heads ecosystem
• Helps users make informed decisions about their security configuration

The documentation addresses the community discussions about documenting these modes in the Heads wiki while appropriately deferring to Purism's detailed documentation for implementation specifics.

Fixes #151.

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[Copilot]

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[tlaurion]

LGTM. Waiting for community input, would merge as is.