docs: external postgres (#316)

Add a how-to page on using an external database.

Companion PRs:
 - Substra/orchestrator#210
 - Substra/substra-backend#658

Fixes FL-649

---------

Signed-off-by: Olivier Léobal <[email protected]>

Author: oleobal

docs/source/operations/howto/external-database.rst

@@ -0,0 +1,44 @@
+************************
+Use an external database
+************************
+
+By default, Substra components use their own integrated postgres databases (one per backend, and one for the orchestrator in centralized mode).
+
+They can be pointed to any PostgreSQL instance (version 11 or better).
+
+The backend and orchestrator use the same structure in their values:
+
+.. code-block:: yaml
+   
+   database:
+     host: my.db.com
+     port: 5432
+     
+     auth:
+      username: my-username
+      password: my-password
+      database: my-substra-db
+
+Or, for improved security, you can create a secret with your database credentials, under the ``DATABASE_PASSWORD`` and ``DATABASE_USERNAME`` keys. Secrets can be `made very secure <https://kubernetes.io/docs/concepts/security/secrets-good-practices/>`_ but this is the basic example:
+
+.. code-block:: yaml
+   
+   apiVersion: v1
+   kind: Secret
+   metadata:
+     name: my-db-secret
+   stringData:
+     DATABASE_PASSWORD: my-password
+     DATABASE_USERNAME: my-username
+
+And then point to it in the values, instead of using username & password:
+
+.. code-block:: yaml
+   
+   database:
+     host: my.db.com
+     port: 5432
+     
+     auth:
+      database: my-substra-db
+      credentialsSecretName: my-db-secret