Add example DAG and documentation for append

[sunank200]

Please describe the feature you'd like to see
In the past, we had a tutorial which illustrated how to use each of our operators/decorators:
https://github.com/astronomer/astro-sdk/blob/be6280df00ccff0d7a1c0dfb099b2065303dbe88/REFERENCE.md

Describe the solution you'd like
Have a reference page per operator/decorator similar to
https://airflow.apache.org/docs/apache-airflow-providers-amazon/stable/operators/ecs.html#howto-operator-ecsoperator

In which we reference parts of an (automated tested) example DAG which illustrates the usage of that operator/decorator.

Many of these use cases already exist in our example DAGs - we should reference them.

• append
  • single example

Acceptance Criteria

• append
  • single example

• All checks and tests in the CI should pass
• Unit tests (90% code coverage or more, once available)
• Integration tests (if the feature relates to a new database or external service)
• Example DAG
• Docstrings in reStructuredText for each of methods, classes, functions and module-level attributes (including Example DAG on how it should be used)
• Exception handling in case of errors
• Logging (are we exposing useful information to the user? e.g. source and destination)
• Improve the documentation (README, Sphinx, and any other relevant)
• How to use Guide for the feature (example)