[DOCS] Add blog post: raster Python UDFs in 1.9.1

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• No: this is a documentation update. The PR name follows the format [DOCS] my subject

What changes were proposed in this PR?

Add a blog post about the raster Python UDFs shipped in Sedona 1.9.1: "Seven Lines of NumPy, 121 Million Pixels".

The post walks through computing NDVI over a Sentinel-2 scene of California's Central Valley end to end: reading the red and NIR band COGs from the public sentinel-cogs bucket with the tiled raster reader (512 px tiles), pairing the bands with a join on tile position, computing NDVI in a two-raster Python UDF that returns a raster via with_bands(), and writing the result back as Cloud Optimized GeoTIFFs with RS_AsCOG and the raster writer. It also covers using rasterio/SciPy inside UDFs, NODATA handling, the same-grid boundary, and how the workload distributes. Cross-links the tiled reader, STAC, and raster metadata posts.

Includes a cover image (five real NDVI output tiles from the run plus the UDF code) and a before/after figure, both generated from the actual outputs.

How was this patch tested?

• Every code snippet in the post was executed against the released artifacts: a fresh venv with pip install apache-sedona==1.9.1 and sedona-spark-shaded-3.5_2.12:1.9.1 / geotools-wrapper:1.9.1-33.5 / hadoop-aws:3.3.4 from Maven Central, reading s3a://sentinel-cogs anonymously.
• All numbers in the post (484 tiles per band, 76 s end-to-end, NDVI ranges) are the measured output of those runs; the written tiles were read back with the raster reader and checked with RS_BandNoDataValue / RS_SummaryStatsAll, and verified to be internally tiled with overviews and deflate compression.
• Both images are rendered from the run's real outputs.
• pre-commit passes, including the manual-stage oxipng hook on both PNGs (idempotent on second run).
• Verified the rendered page with mkdocs serve: images resolve, code highlighting works, and the blog star-CTA hook is appended.

Did this PR include necessary documentation updates?

• Yes, I have updated the documentation.