Docs: Move contributing guide to root and update developer documentation

[HARSHVARANDANI]

Description

This PR makes the root CONTRIBUTING.md the single source of truth for the project's contribution guidelines.

Previously, the root CONTRIBUTING.md only redirected contributors to docs/developer-guide/contribute.md. Since GitHub automatically surfaces the root CONTRIBUTING.md when users open issues and pull requests, contributors would not immediately see the project's contribution guidelines.

This change:

• Moves the complete contribution guide to the root CONTRIBUTING.md.
• Configures MkDocs to include the root CONTRIBUTING.md in the Developer Guide using pymdownx.snippets, eliminating duplicate documentation.
• Configures the pymdownx.snippets base_path so the root file can be referenced from the documentation.

This ensures contributors receive the full guidelines directly on GitHub while maintaining only a single copy of the documentation.

Related issues

• Fixes Enrich root CONTRIBUTING.md and reference it from mkdocs to avoid duplication [Docs] #721

How was this tested?

• Verified that the documentation site builds successfully with:

  uv run mkdocs serve

• Confirmed that the Developer Guide renders the contents of the root CONTRIBUTING.md.

LLM usage

N/A

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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