Generate API docs from code with sphinx autodoc (#2699)

Expanding on the addition of docstrings in code, this adds python module API docs to sphinx documentation.

A developer can preview what this might look like by running this locally:

```
tox -m docs
```

The option `-W` is added to the tox env so that warning messages are considered errors.

Adds doc generation using the above method to the test GitHub workflow to catch documentation problems on PRs.

Some docstrings have been reformatted and fixed to satisfy errors generated with the above setup.

patroni/postgresql/__init__.py

@@ -178,6 +178,7 @@ class Postgresql(object):
         """Returns the monitoring query with a fixed number of fields.
 
         The query text is constructed based on current state in DCS and PostgreSQL version:
+
         1. function names depend on version. wal/lsn for v10+ and xlog/location for pre v10.
         2. for primary we query timeline_id (extracted from pg_walfile_name()) and pg_current_wal_lsn()
         3. for replicas we query pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn(), and  pg_is_wal_replay_paused()
@@ -187,7 +188,8 @@ class Postgresql(object):
         7. if sync replication is enabled we query pg_stat_replication and aggregate the result.
            In addition to that we get current values of synchronous_commit and synchronous_standby_names GUCs.
 
-        If some conditions are not satisfied we simply put static values instead. E.g., NULL, 0, '', and so on."""
+        If some conditions are not satisfied we simply put static values instead. E.g., NULL, 0, '', and so on.
+        """
 
         extra = ", " + (("pg_catalog.current_setting('synchronous_commit'), "
                          "pg_catalog.current_setting('synchronous_standby_names'), "