DOC: add linkcode to docs

[raybellwaves]

Description

This PR adds a [source] button in the API docs which allows readers to jump into the code behind the API docs.

This is currently done in pandas e.g. https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.html#pandas.DataFrame and below. The code is also copied and modified from the pandas repo (https://github.com/pandas-dev/pandas/blob/main/doc/source/conf.py#L637).

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[raybellwaves]

Ready for review.

Tested a few examples and works as expected e.g.

• https://docs.rapids.ai/api/cudf/stable/user_guide/api_docs/api/cudf.series/#cudf.Series now has a source button that links to https://github.com/rapidsai/cudf/blob/branch-24.08/python/cudf/cudf/core/series.py#L418-L3636
• https://docs.rapids.ai/api/cudf/stable/user_guide/api_docs/api/cudf.core.series.datetimeproperties.year/ now has a source button that links to https://github.com/rapidsai/cudf/blob/branch-24.08/python/cudf/cudf/core/series.py#L3750-L3773

[vyasr]

LGTM, thanks!

[vyasr]

/ok to test

[vyasr]

/merge