nldas2 data mdx description added

datasets/nldas2.data.mdx

@@ -1,6 +1,7 @@
 ---
 id: nldas2
 name:  "NLDAS-2 Precipitation Forcing Dataset"
+description: "NLDAS-2 is a surface meteorological analysis and land-surface model dataset running in operations to produce outputs of soil moisture, snow, surface fluxes, streamflow, etc. for drought monitoring and other applications."
 media:
   src: ::file ./nldas-cover.png
   alt: Landsat 8 — OLI image of the Mississippi River below Memphis, Tennessee on September 16, 2023 at near record low water levels, limiting barge shipments, threatening drinking water supplies, agriculture, and ecosystems.y.