RTD: document pin_memory op in Host Memory Pinning
Point the native backend at the standalone pin_memory op (not async-io),
add the pin_handle API, and mirror the tutorial example in DeepNVMe docs.
Signed-off-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
