docs : added screenshot placeholders and walkthrough to plugin health dashboard guide

[tmdeveloper007]

Closes #1104.

Summary of What Has Been Done:
Added a step-by-step walkthrough section and screenshot placeholders to docs/plugin_health_dashboard.md to help new users navigate and interpret the dashboard output.

Changes Made:

• Added a 5-step walkthrough covering: running the dashboard, reading the output structure, interpreting health indicators, taking action on unhealthy plugins, and automating in CI
• Added a health indicators table mapping each state (healthy, missing parser, unregistered) to recommended actions
• Added three screenshot placeholder sections with detailed descriptions of what each visual should show
• Added a CI gate example using the JSON output exit code

Impact it Made:

• Reduces onboarding friction for developers discovering the plugin health dashboard
• Provides clear action items for fixing unhealthy plugins
• Documents the CI integration pattern for automated health checks

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.