doc(spanner): create page for configuration options

[coryan]

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 031dba6f5c7d286642ff685321301f4ffdf8f7fa

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ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Base: 93.98% // Head: 93.97% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (031dba6) compared to base (fe8f7eb).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10199      +/-   ##
==========================================
- Coverage   93.98%   93.97%   -0.01%     
==========================================
  Files        1531     1531              
  Lines      141068   141068              
==========================================
- Hits       132584   132573      -11     
- Misses       8484     8495      +11     

Impacted Files	Coverage Δ
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-1.18%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	84.21% <0.00%> (-0.66%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
google/cloud/storage/parallel_upload.cc	98.28% <0.00%> (-0.35%)	⬇️
google/cloud/completion_queue_test.cc	97.13% <0.00%> (-0.20%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.70% <0.00%> (-0.08%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (+1.00%)	⬆️

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