docs: add docs for DataFrame and Series dunder methods

[shobsi]

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Fixes internal issue 304747371 🦕

[tswast]

Were any of these docstring or examples derived from the pandas documentation? If so, they need to be moved to third_party.

[shobsi]

Were any of these docstring or examples derived from the pandas documentation? If so, they need to be moved to third_party.

DataFrame.__array__ is one case that could be debated. There is no pandas DataFrame.__array__ documentation, so I put BigFrames DataFrame.__array__ directly. There is pandas Series.__array__, so I added BigFrames Series.__array__ in third party. For consistency I kept wording of DataFrame.__array__ close to Series.__array__.

Similarly for bunch of methods like __eq__, __ne__, __gt__ ... there is no pandas documentation but I am adding docstring with code samples directly in our classes which are worded similar to their public versions (eq, ne, gt respectively) for consistency.

Is that acceptable?

[tswast]

adding docstring with code samples directly in our classes which are worded similar to their public versions

Anything derived from third-party content needs to be in the third-party directory.

[shobsi]

adding docstring with code samples directly in our classes which are worded similar to their public versions

Anything derived from third-party content needs to be in the third-party directory.

Done.