Updates the cuDF docs to use the NVIDIA Sphinx theme, following the same direction as rapidsai/rmm#2443.

This removes the local RAPIDS custom CSS/JS injection from the Sphinx and Doxygen docs configuration, switches the cuDF and dask-cuDF Sphinx configs to `nvidia_sphinx_theme`, and updates docs dependencies from `pydata-sphinx-theme` to `nvidia-sphinx-theme`.

## Validation

- Parsed edited Sphinx config files with `ast.parse`.
- Parsed `dependencies.yaml` and generated conda environment YAML files with `yaml.safe_load`.
- Ran commit-time pre-commit hooks, including `RAPIDS dependency file generator`.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #23035

## Description

- uses `NVIDIA RMM` for the documentation project and landing-page
branding
- removes the inactive Twitter/X icon
- uses `NVIDIA RMM Documentation` consistently for HTML, LaTeX, man, and
Texinfo output
- retains the GitHub icon and NVIDIA theme defaults

Follow-up to rapidsai/build-planning#300 and #2443.

## Validation

- Doxygen generation passes
- strict Sphinx HTML build passes with `-n -W --keep-going`
- targeted pre-commit hooks pass

## Checklist

- [x] I am familiar with the [Contributing
Guidelines](https://github.com/rapidsai/rmm/blob/HEAD/CONTRIBUTING.md).
- [x] New or existing tests cover these changes.
- [x] The documentation is up to date with these changes.