Start expanding the docs on initiatives

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

docs/datanommer_datagrepper/datanommer.rst

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+.. _datanommer:
+
+Datanommer
+==========
+
+* Reads-in messages from the bus
+* Stores them into the database
+
+Database tables
+---------------
+
+Here is how the database schema looks like currently:
+
+::
+
+    datanommer=# \dt
+                   List of relations
+     Schema |       Name       | Type  |   Owner
+    --------+------------------+-------+------------
+     public | alembic_version  | table | datanommer
+     public | messages         | table | datanommer
+     public | package          | table | datanommer
+     public | package_messages | table | datanommer
+     public | user             | table | datanommer
+     public | user_messages    | table | datanommer

docs/datanommer_datagrepper/index.rst

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+Datanommer / Datagrepper
+========================
+
+Datanommer
+----------
+
+* Reads-in messages from the bus
+* Stores them into the database
+
+:ref:`datanommer`
+
+Datagrepper
+-----------
+
+* Exposes the messages in the database via an API with different filtering
+  capacity