docs: created samples for load table and create table from schema file

[Mattix23]

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Towards #1069 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/create_table_schema_from_json.py:22, tag bigquery_schema_file_create
• samples/snippets/create_table_schema_from_json.py:35, tag bigquery_schema_file_create
• samples/snippets/load_table_schema_from_json.py:23, tag bigquery_schema_file_load
• samples/snippets/load_table_schema_from_json.py:38, tag bigquery_schema_file_load

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[tswast]

@Mattix23 @thejaredchapman We are getting some type checking failures. I've made some suggested edits that might fix this.

nox > Running session mypy_samples
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/mypy_samples
nox > python -m pip install -e '.[all]'
nox > python -m pip install ipython pytest
nox > python -m pip install mypy==0.910
nox > python -m pip install types-mock types-pytz
nox > python -m pip install typing-extensions
nox > mypy --config-file /tmpfs/src/github/python-bigquery/samples/mypy.ini --no-incremental samples/
samples/snippets/load_table_schema_from_json.py:18: error: Function is missing a return type annotation
samples/snippets/load_table_schema_from_json.py:37: error: Incompatible types in assignment (expression has type "Path", variable has type "str")
samples/snippets/create_table_schema_from_json.py:18: error: Function is missing a return type annotation
samples/snippets/create_table_schema_from_json.py:34: error: Incompatible types in assignment (expression has type "Path", variable has type "str")
Found 4 errors in 2 files (checked 212 source files)
nox > Command mypy --config-file /tmpfs/src/github/python-bigquery/samples/mypy.ini --no-incremental samples/ failed with exit code 1
nox > Session mypy_samples failed.

[tswast]

One minor change, otherwise looking good!