docs: split DataFrame and Series methods into their own pages

[tswast]

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Fixes internal issue 401261155 🦕

[tswast]

Tip from @arwas11 : https://docs.python.org/3/library/collections.html#collections.deque could be faster.

[tswast]

Closing. I'd like to revisit this in future. It's possible this script will be useful, but also possibly not, depending on the plugin changes introduced in googleapis/sphinx-docfx-yaml#415 to address this feature.