Document the News layer's own sources, keys and limits

DATA_SOURCES, the README's layer and key tables, CURRENT-STATE and the
changelog describe the News layer as it now is: a configured news server,
or Core's own reading of GDELT (no key), Liveuamap (optional) and World
News API (Local, free plan backlink credited). .env.example gives the
download sizes and the daily read budget; KNOWN-ISSUES lists what machine
coding gets wrong and which disputed areas are named as themselves.

src/tooling/transitQa.test.mjs

@@ -695,7 +695,7 @@ test('public transit documentation uses courtesy attribution and the approved gr
   );
   assert.ok(
     readme.includes(
-      'Twenty-one layers and map sources. **Nineteen have a keyless path.**',
+      'Twenty-two layers and map sources. **Twenty have a keyless path.**',
     ),
   );
   assert.ok(