Cleaning up inconsistencies in the Manage actions how-to page

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

See https://github.com/canonical/operator/pull/2662#issuecomment-5288007034 for the original discussions.

Some examples:

1.

```python
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)`.

2.

```python
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`.

Start with the Manage Relations how-to pages and review the discussion in pull request #2662. Check the shown examples for undefined or inconsistent variable references, including relation versus event.relation and secret versus secret_contents. Done means the affected examples are internally consistent and accurately reflect the intended API usage.

Written by the indexing model from the issue text.