docs: add obstore quickstart notebook

Companion notebook for the obstore tutorial in PlanetaryComputerDataCatalog.
Walks through reading Planetary Computer data via obstore: authenticated
stores from STAC assets, range reads, multi-range reads, async parallelism,
and handing the store to async-geotiff.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: aboydnw