docs: create a few code snippets as examples for using Spanner Graph using cpp

[bharadwajvr]

Add a few code snippets to illustrate how to use Spanner Graph functionality in Java. These examples resemble other snippets for how various other Spanner features can be used.

Fixes #14659

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This change is 

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 9 region tags.

• google/cloud/spanner/samples/graph_samples.cc:32, tag spanner_create_database_with_property_graph
• google/cloud/spanner/samples/graph_samples.cc:95, tag spanner_insert_graph_data
• google/cloud/spanner/samples/graph_samples.cc:152, tag spanner_insert_graph_data_with_dml
• google/cloud/spanner/samples/graph_samples.cc:197, tag spanner_update_graph_data_with_dml
• google/cloud/spanner/samples/graph_samples.cc:228, tag spanner_update_graph_data_with_graph_query_in_dml
• google/cloud/spanner/samples/graph_samples.cc:250, tag spanner_query_graph_data
• google/cloud/spanner/samples/graph_samples.cc:277, tag spanner_query_graph_data_with_parameter
• google/cloud/spanner/samples/graph_samples.cc:306, tag spanner_delete_graph_data_with_dml
• google/cloud/spanner/samples/graph_samples.cc:336, tag spanner_delete_graph_data

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[dbolduc]

/gcbrun

[scotthart]

/gcbrun

[bharadwajvr]

Gentle reminder: @scotthart

[dbolduc]

/gcbrun

[scotthart]

/gcbrun

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 3.61842% with 293 lines in your changes missing coverage. Please review.

Project coverage is 93.49%. Comparing base (41b1152) to head (f119a1e). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/spanner/samples/graph_samples.cc	3.61%	293 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14660      +/-   ##
==========================================
- Coverage   93.62%   93.49%   -0.13%     
==========================================
  Files        2320     2321       +1     
  Lines      207209   207513     +304     
==========================================
+ Hits       193997   194019      +22     
- Misses      13212    13494     +282     

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