Add common terms and definitions page to specification

[Copilot]

This PR adds a new "Definitions" page to the implementors section that consolidates common terms used throughout the Development Container Specification documentation.

Changes Made

• New page: Added _implementors/definitions.md with comprehensive definitions for key concepts
• Navigation integration: Page appears in the sidebar navigation with index 10
• Configuration updates: Updated _config.yml and .gitignore to properly exclude vendor dependencies

Content Included

The definitions page covers essential terms organized into logical sections:

Core Concepts

• Development Container: Container for application development
• Environment: Logical instance of development containers
• Project Workspace Folder: Search location for devcontainer.json files

Configuration

• devcontainer.json: Main configuration file with location precedence
• devcontainer-feature.json: Feature metadata files

Features

• Development Container Features: Self-contained installation units
• Feature Equality: Rules for determining when features are identical
• Round Stable Sort: Algorithm for deterministic feature ordering

Orchestration & Lifecycle

• Image-based/Dockerfile-based/Docker Compose-based: Different container configurations
• Lifecycle Scripts: Commands that run at specific container stages
• Customizations: Tool-specific configurations

Metadata

• Image Metadata: Container image development context
• Merge Logic: Property combination rules

Testing

✅ Jekyll builds successfully without errors
✅ Page renders correctly with proper formatting
✅ Navigation integration works (appears in sidebar)
✅ All anchor links function properly
✅ Responsive design works on different screen sizes

This provides a centralized reference for developers and implementors to understand key terminology used throughout the specification.

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