docs: add example for a complex multi-platform pypi configuration

[rickeylev]

The core PyPI docs and API reference docs have the basics for setting up a multi-platform
Bazel build, but there's a lot of cross-referencing and reading between the lines necessary.

Create a how to guide specifically on how to do it to better explain the nuances.

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a crucial new documentation guide aimed at simplifying the management of multi-platform PyPI dependencies within a Bazel build environment using rules_python and bzlmod. The guide provides a clear, step-by-step approach to configuring diverse Python environments, including variations in Python version, threading models, operating systems, CPU architectures, and hardware accelerators. By offering detailed explanations and a complete working example, it significantly reduces the complexity and guesswork previously associated with setting up such intricate build configurations.

Highlights

• New How-to Guide: A new documentation guide has been added to explain how to manage complex multi-platform PyPI dependencies using rules_python with bzlmod, addressing the current need for extensive cross-referencing in existing docs.
• Mapping Requirements to Bazel Configs: The guide details the process of mapping requirements files to Bazel configuration settings, including defining custom platforms with pip.default and associating requirements files using pip.parse.requirements_by_platform.
• Custom and Predefined Settings: It covers how to define custom build settings (e.g., for CUDA versions) using string_flag and config_setting, and how to leverage predefined rules_python and @platforms settings for OS, CPU, and threading models.
• Complete Example Provided: A comprehensive example is included, demonstrating the configuration for four different Python 3.14 Linux variations (regular, freethreaded, CUDA 12.9, and ARM/Musl) within BUILD.bazel and MODULE.bazel files.

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[gemini-code-assist]

Code Review

This pull request adds a new how-to guide for managing multi-platform PyPI dependencies. The guide is well-structured and provides a good example. I've found a few typos and a broken command in a code snippet. Addressing these will improve the clarity and correctness of the documentation.

[aignas]

LGTM, but please link to this from pip.parse docs and from the existing pypi documentation so that users have a better chance to discover this.

[rickeylev]

LGTM, but please link to this from pip.parse docs and from the existing pypi documentation so that users have a better chance to discover this.

Done