doc(storage): add custom-dual regions example

[coryan]

Motivated by #8586

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Here is the summary of changes.

You are about to add 1 region tag.

• google/cloud/storage/examples/storage_bucket_samples.cc:142, tag storage_create_bucket_dual_region

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[codecov]

Codecov Report

Merging #8595 (8d99104) into main (5308f1c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #8595   +/-   ##
=======================================
  Coverage   93.42%   93.42%           
=======================================
  Files        1456     1456           
  Lines      123806   123820   +14     
=======================================
+ Hits       115666   115681   +15     
+ Misses       8140     8139    -1     

Impacted Files	Coverage Δ
...e/cloud/storage/examples/storage_bucket_samples.cc	93.84% <100.00%> (+0.22%)	⬆️
google/cloud/pubsub/samples/samples.cc	92.02% <0.00%> (-0.08%)	⬇️
google/cloud/storage/internal/curl_handle.h	82.85% <0.00%> (+2.85%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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[coryan]

I want to double check some things before merging.

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