Improve contribution templates and remove Docker support

[yu-ta-sato]

Summary

• Make the bug and feature request forms welcoming to incomplete and early-stage reports.
• Reduce each issue form to one required free-text field while keeping useful follow-up details optional.
• Simplify the pull request template and fix the Contributing Guide link.
• Remove the unused Docker development files, documentation, security scope, and Dependabot configuration.

The previous issue forms required multiple confirmations and detailed reproduction or environment information before submission, which could discourage new users from reporting problems or sharing ideas.

Related issues

Not applicable.

Testing

• uv run pytest -q — 875 passed.
• uv run pre-commit run --all-files — all hooks passed.
• Parsed both Issue Form YAML files successfully and confirmed each has exactly one required field.
• Checked that active Docker support references were removed; only the Unreleased removal note and historical changelog entries remain.

Documentation

• Removed the Docker Compose instructions from the README.
• Removed Docker images from the security support scope.
• Added the Docker support removal to the Unreleased changelog while preserving historical release notes.

Reviewer notes

The historical Docker entries in older changelog releases are intentionally retained as records of the state at those releases.