docs: document the build variants pixi sets automatically

[hunger]

Pixi fills in some build variants for you rather than requiring them in [workspace.build-variants]: target_platform, and c_stdlib/c_stdlib_version derived from the target platform's system requirements when building against conda-forge. Only the compiler defaults were documented before.

Add a "Variants Pixi Sets for You" section to the build variants tutorial covering the derivation, its conda-forge gating, the bare-subdir defaults, and the Windows/musl/CUDA limitations, and cross-reference it from the manifest reference.

How Has This Been Tested?

By reading it :-)

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.