docs(pubsub): add user guide quickstart for Pub/Sub

[suzmue]

Adds an initial user guide for the Pub/Sub client library.

This is more or less the same example that we have in the google_cloud_pubsub::client documentation.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 96.14%. Comparing base (11a0ea8) to head (4b8eeb2). :warning: Report is 190 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3764   +/-   ##
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  Coverage   96.14%   96.14%           
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  Files         140      140           
  Lines        5442     5442           
=======================================
  Hits         5232     5232           
  Misses        210      210           

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[coryan]

@suzmue what did we decide to do with this PR?

[suzmue]

@suzmue what did we decide to do with this PR?

I was just coming here to abandon it and saw your comment. I think we will want to include the quickstarts directly on cgc.