Moved tutorial to Getting Started and made small wording updates

[zachcasper]

Add GitHub Copilot app to Getting Started and refine the integration docs

Summary

Promotes the GitHub Copilot app walkthrough into the Getting Started guide as a
new sequential part, rewrites it as a step-by-step tutorial, and adds supporting
sidebar and shortcode improvements.

Changes

• New Getting Started part. Moves the Copilot app walkthrough from
  integrations/github-copilot-app/canvas-extension/ to
  getting-started/try-github-copilot-app/ as "4. Use the GitHub Copilot app",
  and renumbers "Hands-on labs" to part 5. Updates the Getting Started overview
  ("four" → "five" parts) and repoints the add-connection next-step button to
  the new guide.
• Rewritten walkthrough. Restructures index.md into numbered steps:
  install the plugin, model the application, configure the Environment, plan,
  deploy, access, clean up, plus branch diff, troubleshooting, and feedback
  sections.
• Integration landing page. Updates integrations/github-copilot-app/_index.md
  terminology (GitHub Copilot app, Radius Canvas extension, Application/
  Environment capitalization), aligns the preview banner, and adds a Next steps
  button linking to the new Getting Started guide.
• Sidebar "New" badge inheritance. Adds a has-new-badge.html partial and
  sidebar-tree logic so a parent section inherits a child's new_badge, with
  SCSS to hide the duplicate badge when the section is expanded. Removes the now
  redundant new_badge from the Integrations index.
• Image shortcode. Extends image.html to support alt and class
  attributes.
• Consistency. Renames "Prerequisites" to "Before you begin" in the
  install-radius and container-platforms guides. Relocates the associated
  screenshots to the new guide directory.