Make CI doctests deterministic

[d-chambers]

Summary

• Replace network-backed doctest examples with local DASCore example patches
• Keep documentation examples executable under pytest doctest
• Update README usage to avoid downloading external example data

Verification

• uv run --no-sources ruff check
• uv run --no-sources ty check
• uv run --no-sources pytest --doctest-modules src tests --cov=dascorepy --cov-report=term-missing --cov-fail-under=100 (Python 3.12)
• same pytest command on Python 3.13
• same pytest command on Python 3.14
• uv run --no-sources great-docs lint

Local notes: great-docs build cannot start a Jupyter kernel in this sandbox because socket creation is blocked; GitHub Actions should be the authoritative docs build.