ppdb.yaml: Fix database name, add browser/ppdb.md

Author: mjuric

browser/ppdb.md

@@ -0,0 +1,7 @@
+---
+layout: schema
+title: Prompt Processing Database
+schema: ppdb
+sort-index: 15
+---
+{{ site.data[page.schema].description }}

python/lsst/sdm/schemas/ppdb.yaml

@@ -1,5 +1,5 @@
 ---
-name: "ApdbSchema"
+name: "PpdbSchema"
 version: "9.1.1"
 description: The Alert Production Database (APDB) contains the catalogs resulting from
   image differencing during nightly Prompt Processing as well as the results of