Cleaning up inconsistencies in the Manage actions how-to page

[tromai]

While working on #2662 , I noticed that there were inconsistencies in the Manage Relations how-to pages.

See #2662 (comment) for the original discussions.

There are some inconsistencies in the examples, which I am planning to address in another PR. Let me know if that is okay.

Some examples:

def _on_db_relation_created(self, event: ops.RelationCreatedEvent):
    if not self.unit.is_leader():
        return
    credentials = self.create_database(event.app.name)
    data = DatabaseProviderAppData(credentials=credentials)
    relation.save(data, event.app)

relation is undefined. I assume we need event.relation.save(data, event.app).

def _update_configuration(self, _: ops.Eventbase):
    ...
    secret_contents = self.model.get_secret(id=secret_id).get_contents(
        refresh=True
    )
    self.push_configuration(
        username=secret['username'],
        password=secret['password'],
    )

I assume secret should be secret_contents.

[benhoyt]

@tromai to update the issue description with the actual changes being proposed, instead of just linking to comments.