feat: moved quickstart/ -> google/cloud/spanner/

[devjgm]

Fixes: https://github.com/googleapis/google-cloud-cpp/issues/3973

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This change is 

[devjgm]

Using a draft PR and asking for a review at the same time is a mixed message.

I thought it only notified reviewers after I clicked the "ready for review" button. I assumed that I was just filling in the reviewers field, and they'd be notified once it was ready.... the github UI for draft PRs is itself a mixed message.

But sorry about that. TIL.

[coryan]

Using a draft PR and asking for a review at the same time is a mixed message.

I thought it only notified reviewers after I clicked the "ready for review" button. I assumed that I was just filling in the reviewers field, and they'd be notified once it was ready....

Hmm... Maybe. I am subscribed to get everything in googleapis/google-cloud-cpp*, so I might be notified too early.

[codecov[bot]]

Codecov Report

Merging #1479 into master will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1479      +/-   ##
==========================================
- Coverage   95.61%   95.60%   -0.02%     
==========================================
  Files         194      192       -2     
  Lines       15919    15934      +15     
==========================================
+ Hits        15221    15233      +12     
- Misses        698      701       +3     

Impacted Files	Coverage Δ
google/cloud/spanner/internal/log_wrapper.h	75.00% <0.00%> (-9.62%)	:arrow_down:
google/cloud/spanner/internal/polling_loop.h	83.33% <0.00%> (-9.53%)	:arrow_down:
...loud/spanner/internal/partial_result_set_resume.cc	90.90% <0.00%> (-4.55%)	:arrow_down:
...loud/spanner/internal/partial_result_set_source.cc	93.24% <0.00%> (-1.36%)	:arrow_down:
...ud/spanner/integration_tests/client_stress_test.cc	81.48% <0.00%> (-0.93%)	:arrow_down:
google/cloud/spanner/keys.h	100.00% <0.00%> (ø)
google/cloud/spanner/internal/session.h	100.00% <0.00%> (ø)
google/cloud/spanner/session_pool_options.h	100.00% <0.00%> (ø)
google/cloud/spanner/internal/transaction_impl.h	100.00% <0.00%> (ø)
...oogle/cloud/spanner/internal/instance_admin_stub.h
... and 6 more

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