doc(bigtable): how to mock the Data API

[dbolduc]

Fixes #9308

Add a dedicated document that walks through how to mock Table::ReadRows.

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This change is 

[google-cloud-cpp-bot]

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For commit: ff01cd4c6f590f5d76f1b6e6713bf315584e3a02

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[codecov]

Codecov Report

Merging #9415 (ff01cd4) into main (c23bcf2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9415   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files        1484     1485    +1     
  Lines      136115   136154   +39     
=======================================
+ Hits       128779   128820   +41     
+ Misses       7336     7334    -2     

Impacted Files	Coverage Δ
google/cloud/bigtable/mocks/mock_data_connection.h	100.00% <ø> (ø)
...gle/cloud/bigtable/examples/howto_mock_data_api.cc	100.00% <100.00%> (ø)
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️
...sub/internal/batching_publisher_connection_test.cc	99.41% <0.00%> (-0.20%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.72% <0.00%> (+0.56%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.75% <0.00%> (+0.74%)	⬆️

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