Add comprehensive user guide documentation

[Copilot]

Adds user guide at docs/guide/guide.md covering ReqStream usage, YAML format specification, and common workflows.

Contents

• Introduction: Overview of ReqStream's purpose and capabilities
• Installation: Global and local tool installation via .NET CLI
• Requirements File Format: Complete YAML specification including:
  • Hierarchical sections and subsections
  • Requirement IDs, titles, and child relationships
  • Test mappings (inline and separate)
  • File includes with automatic section merging
• Command-Line Interface: Full option reference with usage patterns
• Exporting: Requirements reports and trace matrix generation
• FAQ: 30+ questions covering usage, format, mapping, and troubleshooting

Example YAML Structure

sections:
  - title: "System Security"
    requirements:
      - id: "SYS-SEC-001"
        title: "The system shall support credentials authentication."
        children:
          - "AUTH-001"

  - title: "User Authentication"
    requirements:
      - id: "AUTH-001"
        title: "All requests shall have their credentials authenticated."
        tests:
          - "Credentials_Valid_Allowed"
          - "Credentials_Invalid_Refused"

includes:
  - additional_requirements.yaml

mappings:
  - id: "AUTH-001"
    tests:
      - "Credentials_Missing_Refused"

Changes

• Created docs/guide/guide.md (734 lines)
• Added "Pandoc" to .cspell.json dictionary

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Feature]: User Guide document</issue_title>
<issue_description>### Problem Statement

The product should have a user guide.

Proposed Solution

A detailed user guide should be created in Markdown as docs/guide/guide.md.

It should have at least the following sections:

• Introduction
  • What is it
  • How should it be used
  • Key features
• Prerequisites
• Requirements File Format
• Exporting
• FAQ

Alternatives Considered

No response

Additional Context

No response

Priority

Medium - Would improve workflow

Contribution

• I would be willing to submit a PR for this feature

Code of Conduct

• I agree to follow this project's Code of Conduct</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature]: User Guide document #39

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