[AIRFLOW-XXX] Add How-To-Guide to GCP PubSub

[TobKed]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-XXX
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Description

• Here are some details about my PR, including screenshots of any UI changes:

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":
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  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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6497 into master will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6497      +/-   ##
==========================================
- Coverage   83.96%   83.65%   -0.32%     
==========================================
  Files         635      635              
  Lines       36678    36679       +1     
==========================================
- Hits        30797    30684     -113     
- Misses       5881     5995     +114

Impacted Files	Coverage Δ
airflow/providers/google/cloud/sensors/pubsub.py	94.28% <ø> (ø)	⬆️
airflow/providers/google/cloud/operators/pubsub.py	90.69% <ø> (ø)	⬆️
...viders/google/cloud/example_dags/example_pubsub.py	0% <ø> (ø)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
airflow/models/taskinstance.py	93.28% <0%> (-0.51%)	⬇️
airflow/lineage/backend/atlas/__init__.py	87.5% <0%> (ø)	⬆️
... and 10 more

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[potiuk]

Please rebase @TobKed :)

[TobKed]

@potiuk rebased and fixed links due to #6476