docs(samples): add quickstart sample

[stephaniewang526]

Fixes b/176624168

[snippet-bot]

Here is the summary of changes.

You added 1 region tag.

• samples/snippets/src/main/java/com/example/bigqueryreservation/QuickstartSample.java:19, tag bigqueryreservation_quickstart

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[codecov]

Codecov Report

Merging #302 (f8c0226) into master (8ec3351) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #302   +/-   ##
=========================================
  Coverage     82.96%   82.96%           
  Complexity      171      171           
=========================================
  Files             6        6           
  Lines          1168     1168           
  Branches         22       22           
=========================================
  Hits            969      969           
  Misses          172      172           
  Partials         27       27           

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