doc(common): add overview section

[coryan]

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: f01421bc62ef681d3207d8cdb23084cb0bb3226a

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• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

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

[codecov]

Codecov Report

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

Coverage data is based on head (f01421b) compared to base (71b11b6).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10193      +/-   ##
==========================================
- Coverage   93.97%   93.97%   -0.01%     
==========================================
  Files        1526     1526              
  Lines      140600   140600              
==========================================
- Hits       132130   132125       -5     
- Misses       8470     8475       +5     

Impacted Files	Coverage Δ
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.15% <0.00%> (-0.51%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.70% <0.00%> (-0.08%)	⬇️
...sub/internal/batching_publisher_connection_test.cc	99.62% <0.00%> (+0.18%)	⬆️

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[scotthart]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)