Prepare UrbanPy 0.3 for community prerelease - #87

Outcome

Gives UrbanPy 0.3 an accurate public, contributor, citation, support, governance, and security surface suitable for community prerelease review.

What changed

• replaces obsolete Python 3.6, Ubuntu 18.04, macOS Catalina, compilation, and PowerShell guidance
• documents the uv/Trunk contributor path and deterministic/live/Docker test boundaries
• adds SECURITY.md, SUPPORT.md, and GOVERNANCE.md
• enables GitHub private vulnerability reporting and links the confidential form
• adds a structured 0.3 changelog and valid CITATION.cff with the published reference DOI
• modernizes the Sphinx landing, install, contribution, and support pages
• adopts PEP 639 metadata with License-Expression: GPL-3.0-only
• distinguishes UrbanPy's software license from provider data licenses
• packages all community/security/citation files in the sdist

Verification

• 56 hermetic tests passed, 2 live/Docker tests deselected
• warning-as-error Sphinx build passed
• Trunk checked 121 files with no issues
• CFF 1.2 schema validation passed
• wheel metadata verified as Metadata-Version 2.4 and GPL-3.0-only
• wheel and sdist built successfully; source contents inspected
• private vulnerability reporting API verified enabled: true

Stack

Base: #85
Roadmap: #61
Parent: #71

Closes #86