Add OSS hygiene scaffolding

[slusset]

Summary

• add issue templates, a code of conduct, and MkDocs documentation scaffolding
• add a GitHub Pages docs workflow and docs dependency group managed through uv
• configure branch protection on main to require CI checks, one approval, and conversation resolution

Verification

• uv lock
• uv sync --locked --group docs
• uv run --group docs mkdocs build --strict
• uv run pytest tests/unit -v --tb=short
• uv run ruff check src/ tests/
• uv run ruff format --check src/ tests/
• uv run mypy src/ehr_writeback/core/

Notes

• CONTRIBUTING.md was already present and already satisfied the issue's dev setup requirement.
• Main branch protection was applied directly via GitHub API as part of this work.