doc: improve instructions for WORKSPACE files

[coryan]

We never explained that users need to load a Starlark function to use
http_archive(). Also fix the documentation to use the current release,
this will get "stale", but would be still functional and we warn users
to update the version and SHA256 in the comments.

Since I was fixing this I also updated the INSTALL.md file (and its
generator) with the changes from #1252, thanks to @antfitch for proposing
these improvements.

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This change is 

[codecov]

Codecov Report

Merging #1254 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #1254 +/- ## ========================================== - Coverage 94.87% 94.83% -0.04%  ========================================== Files 181 181 Lines 13794 13792 -2 ========================================== - Hits 13087 13080 -7  - Misses 707 712 +5

Impacted Files	Coverage Δ
...loud/spanner/internal/partial_result_set_resume.cc	90.9% <0%> (-4.55%)	⬇️
google/cloud/spanner/internal/polling_loop.h	91.89% <0%> (-2.71%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	81.57% <0%> (-1.76%)	⬇️
google/cloud/spanner/internal/spanner_stub.cc	68.47% <0%> (-1.09%)	⬇️
google/cloud/spanner/value.h	92.8% <0%> (-0.11%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.37% <0%> (ø)	⬆️

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