feat(ansible): add mnemosyne db and update ouranos documentation

- Configure mnemosyne database credentials in ansible inventory
- Update postgresql playbook to provision user and database
- Add setup instructions and DB list to documentation

ansible/inventory/host_vars/portia.incus.yml

@@ -50,6 +50,9 @@ periplus_db_password: "{{ vault_periplus_db_password }}"
 daedalus_db_name: daedalus
 daedalus_db_user: daedalus
 daedalus_db_password: "{{ vault_daedalus_db_password }}"
+mnemosyne_db_name: mnemosyne
+mnemosyne_db_user: mnemosyne
+mnemosyne_db_password: "{{ vault_mnemosyne_db_password }}"
 
 # PostgreSQL admin password
 postgres_password: "{{ vault_postgres_password }}"