GitHub security workshop: Add workshop structure, prerequisites, and safe lab setup

Goal

Create the entry point and setup exercise for a dedicated 2–3 hour GitHub security workshop built around the pets application. Learners should know before starting which repository visibility, GitHub plan, organization role, and optional Copilot access each exercise requires.

Scope

Add the workshop overview, navigation, timing, learning objectives, template-repository setup, and a capability preflight. Establish a safety contract for intentional vulnerabilities and secret-scanning demonstrations: use only documented test values, isolate lab branches, and provide reset instructions.

Acceptance criteria

• content/security/README.md introduces the workshop, audience, duration, outcomes, and exercise sequence.
• content/security/0-setup.md guides creation of a fresh public repository from the template and validates the app.
• A capability matrix distinguishes features available for public repositories from features requiring organization, enterprise, or administrative access.
• Copilot is identified as optional for the remediation exercise, with a manual path available.
• Learners create or identify a disposable lab repository and branch strategy before introducing vulnerable examples.
• The workshop explicitly prohibits real credentials and explains how intentional findings will be cleaned up.
• Previous/next navigation and the root workshop selection page include the new track.
• All links and setup steps are validated from a fresh template repository.

Start with the existing workshop structure and add content/security/README.md and content/security/0-setup.md, then trace the root workshop selection and previous/next navigation for the new track. Validate setup from a fresh template repository, including the capability matrix, disposable lab and branch guidance, optional Copilot path, safety contract, and link correctness.