Add Sampling Design recipe documentation

[cdanielw]

Summary

• Add a beginner-facing cookbook guide for the Sampling Design recipe.
• Document the AOI, stratification, anticipated proportions, sample allocation, sample arrangement, export behavior and output fields.
• Add current interface screenshots.
• Add Sampling Design to the cookbook navigation.

Validation

• .venv/bin/sphinx-build -b html docs/source docs/build/html -w warnings.txt
• .venv/bin/python tests/check_warnings.py
• git diff --check
• Reviewed the rendered Sampling Design page locally.

The Sphinx build succeeds with the existing allow-listed warning that docs/source/setup/create_github.rst is not included in a toctree.