docs: use c.g.com/cpp mocking guide

[dbolduc]

Fixes #11868

Prefer cloud.google.com/cpp over googlapis.dev/cpp

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This change is 

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f9cc920) 93.67% compared to head (65b4b03) 93.67%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #11869   +/-   ##
=======================================
  Coverage   93.67%   93.67%           
=======================================
  Files        1838     1838           
  Lines      166116   166116           
=======================================
+ Hits       155601   155605    +4     
+ Misses      10515    10511    -4     

Impacted Files	Coverage Δ
...den/v1/mocks/mock_golden_kitchen_sink_connection.h	100.00% <ø> (ø)
...lden/v1/mocks/mock_golden_thing_admin_connection.h	100.00% <ø> (ø)
generator/internal/mock_connection_generator.cc	63.63% <ø> (ø)
...gle/cloud/pubsublite/mocks/mock_admin_connection.h	4.76% <ø> (ø)

... and 4 files with indirect coverage changes

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