[AIRFLOW-3995][WIP] Extract code from docs

[mik-laj]

Make sure you have checked all steps below.

Jira

• https://issues.apache.org/jira/browse/AIRFLOW-3995

Description

I extracted fragments of codes to separate files. This allows us to test them automatically. Some examples were incorrect, so I fixed them.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[potiuk]

@mik-laj -> you are my documentation hero ;)

Love the change.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@68e8c99). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master      #77   +/-   ##
=========================================
  Coverage          ?   10.74%           
=========================================
  Files             ?      484           
  Lines             ?    30234           
  Branches          ?        0           
=========================================
  Hits              ?     3250           
  Misses            ?    26984           
  Partials          ?        0

Impacted Files	Coverage Δ
airflow/example_dags/tutorial.py	0% <ø> (ø)
...ample_dags/example_branch_python_dop_operator_3.py	0% <ø> (ø)
...w/example_dags/example_latest_only_with_trigger.py	0% <ø> (ø)
...low/example_dags/example_trigger_controller_dag.py	0% <ø> (ø)
airflow/plugins_manager.py	85.84% <ø> (ø)
airflow/example_dags/example_latest_only.py	0% <ø> (ø)
airflow/example_dags/example_lineage.py	0% <0%> (ø)
airflow/example_dags/example_default_args.py	0% <0%> (ø)
airflow/example_dags/example_documentation.py	0% <0%> (ø)
airflow/example_dags/example_bash_operator.py	0% <0%> (ø)
... and 4 more

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