[yaml] Add examples for Spanner IO in YAML

[reeba212]

Added examples with explanatory comments for reading from and writing to a spanner table

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All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 57.31%. Comparing base (2573106) to head (92f8d24). Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #32288      +/-   ##
============================================
- Coverage     57.32%   57.31%   -0.02%     
  Complexity     1474     1474              
============================================
  Files           965      965              
  Lines        153454   153454              
  Branches       1076     1076              
============================================
- Hits          87974    87956      -18     
- Misses        63276    63294      +18     
  Partials       2204     2204              

Flag	Coverage Δ
python	81.50% <ø> (-0.03%)	:arrow_down:

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R: @Polber

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