[REVIEW] Change cudf docs theme to pydata theme

[galipremsagar]

Fixes: #7098

This PR changes our docs theme to pydata theme: https://github.com/pydata/pydata-sphinx-theme, rest of the additional changes include:

• Revamps cudf docs theme to use pydata theme, and made changes to generate separate html pages for each API thus docs are not going to take ~30s to load anymore, you should see the doc page for each API load almost instantly.
• Replaced sphinx_rtd_theme usages with pydata-sphinx-theme.
• Removed copybutton being used in code samples, pydata-sphinx-theme has this support inbuilt.
• Introduced rapids color scheme 🟪 css styling into pydata-sphinx-theme.
• Removed some dead-code related to markdown in docs. - We no longer use markdown in docs.
• Moved all Class-related docstrings into Class declarations rather than in their __init__ methods, to make the docstrings be uniform for autoclass_content to work correctly.
• Removed the rapids css which is now not needed: "https://docs.rapids.ai/assets/css/custom.css"
• Fixed reference errors in multipe places in docstrings; for ex: replaced cudf.core.series.Series.factorize with cudf.Series.factorize.
• Make top-level import of StringIndex possible, previously we cannot do cudf.StringIndex, this PR enables it.
• Enable top-level imports for pivot & unstack: cudf.pivot & cudf.unstack.

For reviewing purposes, a demo live url to docs is here: https://6111734dedccb71ae0db66dc--docs-rapids-ai.netlify.app/api/cudf/stable/

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[quasiben]

Thanks @galipremsagar ! And thank you @kkraus14 and @isVoid for the reviews.

With the new theme and multiple pages we may want to consider restructuring a bit more. For example, the Basics heading no longer makes much sense and more interesting topics, like GDS, are not easily visible. Again, for another PR/discussion.

[quasiben]

@gpucibot merge

[charlesbluca]

Think this is still blocked by rapidsai/integration#336 and an ops approval

[ajschmidt8]

just waiting for the changes in rapidsai/integration#336 to be published and then this can be approved and merged.

[ajschmidt8]

GitHub is having issues which is probably why this didn't merge. Let's try another merge comment. If not, we'll wait a bit until their problems are resolved.

https://www.githubstatus.com/incidents/rmfrw9dfbtbp

[ajschmidt8]

@gpucibot merge

[ajschmidt8]

had some doc build issues. this should fix it. i tested it locally: rapidsai/integration#338

[ajschmidt8]

new docs are live: https://docs.rapids.ai/api/cudf/nightly/