Improve contributor onboarding and prepare good-first issues

[httpdss]

Summary

Improve contributor onboarding and create a set of clearly scoped good-first issues, especially around the Python backend rewrite and documentation.

Scope

• Update CONTRIBUTING docs with local setup, test commands, coding expectations, and PR checklist.
• Add issue templates for bugs, docs improvements, and Python backend test feedback.
• Identify or create small tasks suitable for good first issue / help wanted labels.
• Link contribution guidance from README.

Suggested good-first issue areas

• Documentation examples and screenshots.
• Python backend test coverage.
• Install guide verification on specific distros.
• UI copy/alt text/accessibility improvements.
• Small troubleshooting entries based on common setup failures.

Acceptance criteria

• New contributors can set up the project and run tests from the docs.
• Issue templates guide users to include useful environment and collectd/RRD details.
• At least 5 small contribution opportunities are identified or created.
• README links to the contribution guide.

[github-actions]

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