[docs] Add mascot artwork to README and docs (#121)

[coisa]

Summary

Adds the project mascot artwork to the repository README and documentation so the project gains a light visual identity without changing the technical structure.

Included

• add the mascot image as a tracked documentation asset under docs/_static/mascot.png
• display the mascot near the top of README.md
• display the mascot on the Sphinx documentation landing page in docs/index.rst
• add a changelog entry for the documentation-facing change

Validation

• php bin/dev-tools changelog:check
• git diff --check
• reviewed the rendered Markdown and reStructuredText source for lightweight placement

Notes

This is intentionally a small personality pass. It does not redesign the README, the docs theme, or any workflow behavior.

Closes #121

[github-actions]

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• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-122/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-122/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-122/metrics/