Document code from example DAGs

[kaxil]

Instead of hard-coding the docs in the GETTING_STARTED.md, we should use the following code to extract the code from example DAGs so it doesn't go out of date.

```{literalinclude} ../../example_dags/example_amazon_s3_postgres.py
:language: python
:start-after: "# [START howto_s3_loadfile]"
:end-before: "# [END howto_s3_loadfile]"
```

where example_amazon_s3_postgres.py can be as follows, notice the # [START howto_s3_loadfile] and # [END howto_s3_loadfile] marker:

import os
from datetime import datetime, timedelta

from airflow.models import DAG
from pandas import DataFrame

from astro import sql as aql
from astro.files import File
from astro.sql.table import Table


@aql.transform
def sample_create_table(input_table: Table):
    return "SELECT * FROM {{input_table}} LIMIT 10"


@aql.dataframe(identifiers_as_lower=False)
def my_df_func(input_df: DataFrame):
    print(input_df)


# [START howto_s3_loadfile]
s3_bucket = os.getenv("S3_BUCKET", "s3://tmp9")

dag = DAG(
    dag_id="example_amazon_s3_postgres",
    start_date=datetime(2019, 1, 1),
    max_active_runs=3,
    schedule_interval=timedelta(minutes=30),
    default_args={"retries": 1}
)

with dag:
    my_homes_table = aql.load_file(
        input_file=File(path=f"{s3_bucket}/homes.csv"),
        output_table=Table(conn_id="postgres_conn"),
    )
    # [END howto_s3_loadfile]

    sample_table = sample_create_table(my_homes_table)
    my_df_func(sample_table)
    aql.cleanup()

This task involves:

• [ ] Making changes to example DAGs (add dags / tasks) in [example_dags directory](https://github.com/astronomer/astro-sdk/tree/main/example_dags directory) to cover all the scenarios in the GETTING_STARTED.md

[kaxil]

https://github.com/astronomer/astro-sdk/issues/530 should be completed first

[kaxil]

@utkarsharma2 @sunank200 Thoughts whether this is still needed or already taken care of by https://github.com/astronomer/astro-sdk/issues/584?

[sunank200]

@kaxil I think readthedocs has this as part of Simple ETL workflow.