DOC: Add code of conduct, issue/PR templates, and CODEOWNERS

Problem

The repository is missing most standard community health files:

The missing issue templates connect directly to the backlog-triage issue in this milestone: an unlabelled, decade-old backlog is what a repository with no triage scaffolding accumulates. A bug template that asks for versions of Python, NumPy, numba and SciPy would resolve a large share of triage questions before a maintainer reads the issue. CODEOWNERS is worth its own thought given that two contributors account for roughly 70% of recent human commits — routing reviews by subpackage at least makes the concentration visible.

Acceptance criteria

• CODE_OF_CONDUCT.md, aligned with any QuantEcon org-level policy
• Issue templates for bug report and feature request, capturing the version matrix
• A pull request template referencing the contributing guide and the test/lint expectations
• .github/CODEOWNERS routing reviews by subpackage
• CONTRIBUTING.md covers the local dev loop (test, lint, docs build) inline rather than only linking out

From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).

Review the existing CONTRIBUTING.md and the repository's current community-health files, then compare the requested CODE_OF_CONDUCT.md, .github issue and pull-request templates, and .github/CODEOWNERS with the acceptance criteria. Confirm the Python, NumPy, numba, and SciPy version questions plus test, lint, and docs-build expectations. Done means all listed files exist, CODEOWNERS routes reviews by subpackage, and CONTRIBUTING.md documents the local development loop inline.