docs: add custom attribution tutorial

[harshkumar18-bzz]

Issues

• Closes: Docs: Generate attribution with custom template tutorial #722

Changes

• Add an end-to-end tutorial for customizing the attribution HTML template through .scancode/templates/attribution.html.
• Document the current Django template context (project, packages, and licenses) and show a small branding customization.
• Explain archive layout, attribution download steps, troubleshooting, and the precedence of the Project Settings template.
• Link the tutorial from the documentation overview and attribution output reference.

This follows the current implementation and avoids the invalid resources-based example proposed in the earlier, self-closed #1968.

Validation

• doc8 --max-line-length 100 --ignore-path docs/_build/ --quiet docs/
• Full Sphinx HTML build succeeds. The stricter warnings-as-errors run reaches and renders the new page; it reports only pre-existing/environment warnings for unavailable external intersphinx inventories and the existing undefined changelog label.

Checklist

• I have read the contributing guidelines
• I have linked an existing issue above
• No unit test is needed for this documentation-only change
• Every changed line was reviewed against the current implementation

AI assistance disclosure

OpenAI Codex was used to inspect the issue and related history, check the current implementation, draft the documentation change, run validation, and prepare this pull request.