docs: use autosummary to split documentation pages

[tswast]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[tswast]

e2e failures:

FAILED tests/system/small/test_dataframe_io.py::test_to_gbq_if_exists_is_replace
FAILED tests/system/small/test_dataframe_io.py::test_to_gbq_if_exists_is_append
FAILED tests/system/small/test_dataframe.py::test_to_gbq_table_labels - googl...
3 failed, 3502 passed, 33 skipped, 42 xfailed, 3 xpassed, 523 warnings in 751.92s (0:12:31)
nox > Command py.test --quiet -n=20 --timeout=600 --junitxml=small_prerelease_3.13_sponge_log.xml --cov=bigframes --cov=tests/system/small --cov-append --cov-config=.coveragerc --cov-report=term-missing --cov-fail-under=0 tests/system/small --ignore=tests/system/small/functions/test_remote_function.py failed with exit code 1
nox > Session system_prerelease failed.

I believe these are the known issue of SQLGlot incompatibility.

[sycai]

Thanks for the cleaning!