chore: Issue367 get repo working with standard infrastructure

[jimfulton]

Fixes #367

Nox is working at the top and in samples.

Some unit tests fail and system tests hang, but I assume that's independent of the repo setup.

I created this by cheery-picking commits from #381, excluding the commit that checked in the result of running owlbot, so the statements above assume that owlbot is run manually.

Next steps:

• @tswast review of current status
• Check in files and changes from running owlbot
• Get unit and system tests passing.

[snippet-bot]

Here is the summary of changes.

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• docs/intro.rst:33, tag bigquery_pandas_gbq_read_gbq_simple
• docs/intro.rst:63, tag bigquery_pandas_gbq_to_gbq_simple
• docs/reading.rst:12, tag bigquery_pandas_gbq_read_gbq_simple
• docs/reading.rst:43, tag bigquery_pandas_gbq_read_gbq_legacy
• docs/writing.rst:12, tag bigquery_pandas_gbq_to_gbq_simple

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• docs/source/intro.rst:33, tag bigquery_pandas_gbq_read_gbq_simple
• docs/source/intro.rst:63, tag bigquery_pandas_gbq_to_gbq_simple
• docs/source/reading.rst:12, tag bigquery_pandas_gbq_read_gbq_simple
• docs/source/reading.rst:43, tag bigquery_pandas_gbq_read_gbq_legacy
• docs/source/writing.rst:12, tag bigquery_pandas_gbq_to_gbq_simple

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[jimfulton]

IDK why circleci is running. I'm guessing changes are needed outside this repo.

[tswast]

@jimfulton To keep the diff smaller, could we migrate the docs to our standard directory structure in a separate PR before we migrate everything else?

Also, as discussed offline, we'll need at least a few tests to ensure our package works from a conda environment with conda-installed dependencies. Currently these are done manually in Circle, but I think we can move those to the noxfile with https://nox.thea.codes/en/stable/tutorial.html?highlight=conda#testing-with-conda -- That also probably warrants a separate PR before migrating to our full templates.