feat: moved INSTALL.md -> doc/packaging.md

[devjgm]

This is the final step fixing #1309

INSTALL.md has been repurposed and renamed to doc/packaging.md. I also renamed the ci/test-readme directory to ci/test-markdown, which is more general.

You can preview the rendered markdown here: https://github.com/devjgm/google-cloud-cpp-spanner/blob/mv-install/doc/packaging.md

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This change is 

[coryan]

Reviewed 9 of 9 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #1341 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   94.64%   94.69%   +0.04%     
==========================================
  Files         173      173              
  Lines       13644    13633      -11     
==========================================
- Hits        12914    12910       -4     
+ Misses        730      723       -7

Impacted Files	Coverage Δ
...anner/integration_tests/client_integration_test.cc	98.5% <0%> (-0.01%)	⬇️
google/cloud/spanner/database_admin_connection.h
...oogle/cloud/spanner/internal/instance_admin_stub.h
...oogle/cloud/spanner/internal/database_admin_stub.h	100% <0%> (ø)
google/cloud/spanner/instance_admin_connection.h	100% <0%> (ø)
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.97% <0%> (ø)	⬆️
google/cloud/spanner/samples/samples.cc	89.62% <0%> (+0.28%)	⬆️
google/cloud/spanner/internal/log_wrapper.h	84.61% <0%> (+9.61%)	⬆️

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