docs: add how-to guide and include API reference

[tswast]

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Fixes #14
Fixes #27
🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 14 region tags.

• docs/usage.rst:13, tag bigquery_pandas_date_create
• docs/usage.rst:26, tag bigquery_pandas_date_as_datetime
• docs/usage.rst:35, tag bigquery_pandas_date_sub
• docs/usage.rst:44, tag bigquery_pandas_date_add_offset
• docs/usage.rst:57, tag bigquery_pandas_time_create
• docs/usage.rst:66, tag bigquery_pandas_time_as_timedelta
• docs/usage.rst:79, tag bigquery_pandas_combine_date_time
• samples/snippets/pandas_date_and_time.py:17, tag bigquery_pandas_date_create
• samples/snippets/pandas_date_and_time.py:26, tag bigquery_pandas_date_as_datetime
• samples/snippets/pandas_date_and_time.py:31, tag bigquery_pandas_date_sub
• samples/snippets/pandas_date_and_time.py:37, tag bigquery_pandas_date_add_offset
• samples/snippets/pandas_date_and_time.py:44, tag bigquery_pandas_time_create
• samples/snippets/pandas_date_and_time.py:49, tag bigquery_pandas_time_as_timedelta
• samples/snippets/pandas_date_and_time.py:59, tag bigquery_pandas_combine_date_time

You are about to delete 8 region tags.

• samples/snippets/pandas_date_and_time.py:17, tag bigquery_date_create
• samples/snippets/pandas_date_and_time.py:26, tag bigquery_date_as_datetime
• samples/snippets/pandas_date_and_time.py:31, tag bigquery_date_sub
• samples/snippets/pandas_date_and_time.py:37, tag bigquery_date_do
• samples/snippets/pandas_date_and_time.py:44, tag bigquery_time_create
• samples/snippets/pandas_date_and_time.py:49, tag bigquery_time_as_timedelta
• samples/snippets/pandas_date_and_time.py:54, tag bigquery_combine_date_time
• samples/snippets/pandas_date_and_time.py:60, tag bigquery_combine2_date_time

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[tswast]

Python 3.10 failure is expected. No wheels yet for pandas