Add example of filtering a repeat table

Recommendation from a user:

```
repeats = client.submissions.get_table(form_id='simple_repeat', table_name='Submissions.observation', filter="$root/Submissions/__system/reviewState ne 'rejected'")

obs_df = pd.json_normalize(data=repeats['value'], sep='/')

obs_df.head(3)
```

Use the supplied pyodk and pandas snippet as the starting point, then locate the repository's documentation or examples area for the appropriate place to add it. Done means the filtering example is included with enough context to show how repeat-table results are normalized and inspected.

Written by the indexing model from the issue text.