Add IzzyOnDroid to APK alternatives; keep the catalogue's index file

Reducing an index no longer deletes apk-catalog/data/index-v2.json, which
the catalogue build reads. Drop the measurement log from docs.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1,7 +1,6 @@
 .DS_Store
 __pycache__/
 apk-catalog/data/cache/
-apk-catalog/data/index-v2.json*
+apk-catalog/data/index-v2*.json*
 compat-db/.env.lakebed.server
 compat-db/.lakebed/
-apk-catalog/data/index-v2.archive.json*