docs(samples): add query external sheet temp table

[pmakani]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #659 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #659      +/-   ##
============================================
- Coverage     81.49%   81.35%   -0.15%     
  Complexity     1226     1226              
============================================
  Files            77       77              
  Lines          6220     6220              
  Branches        691      685       -6     
============================================
- Hits           5069     5060       -9     
- Misses          792      797       +5     
- Partials        359      363       +4     

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/google/cloud/bigquery/BigQueryImpl.java	82.54% <0.00%> (-1.05%)	74.00% <0.00%> (ø%)
...ogle/cloud/bigquery/WriteChannelConfiguration.java	86.36% <0.00%> (-0.46%)	46.00% <0.00%> (ø%)
...om/google/cloud/bigquery/LoadJobConfiguration.java	92.83% <0.00%> (-0.38%)	56.00% <0.00%> (ø%)
...m/google/cloud/bigquery/QueryJobConfiguration.java	89.80% <0.00%> (-0.28%)	66.00% <0.00%> (ø%)

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