docs(samples): add a new sample for parallel append

[yayi-google]

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Fixes #<issue_number_goes_here> ☕️

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/src/main/java/com/example/bigquerystorage/ParallelWriteCommittedStream.java:19, tag bigquerystorage_jsonstreamwriter_parallelcommitted

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[codecov]

Codecov Report

Merging #863 (b93679c) into master (3a942cd) will increase coverage by 0.39%.
The diff coverage is 95.71%.

@@             Coverage Diff              @@
##             master     #863      +/-   ##
============================================
+ Coverage     80.24%   80.64%   +0.39%     
- Complexity      979     1005      +26     
============================================
  Files            74       76       +2     
  Lines          5290     5430     +140     
  Branches        409      420      +11     
============================================
+ Hits           4245     4379     +134     
- Misses          872      878       +6     
  Partials        173      173              

Impacted Files	Coverage Δ	Complexity Δ
...cloud/bigquery/storage/v1beta2/StreamWriterV2.java	94.95% <94.95%> (ø)	23.00 <23.00> (?)
...oud/bigquery/storage/v1beta2/StreamConnection.java	100.00% <100.00%> (ø)	3.00 <3.00> (?)

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[yayi-google]

The only one failing test is unrelated to this CL, and is removed in a separate pull request.

[stephaniewang526]

please change the subject and initial commit from feat: ... to docs(samples): ... since this is not a feat and it will go into the release notes.