feat(config): add --describe flag to pixi config list

[ZachFlint]

Adds an annotated TOML-style listing of every available configuration option with its type, default, and a short explanation. Keys that are currently set print uncommented with their value; the rest are emitted as commented stubs showing the default.

--describe [KEY] describes one key. --describe --json emits a structured array suitable for tooling.

Closes #1457.

Description

Fixes #{issue}

How Has This Been Tested?

• Added unit tests in pixi_config cross-checking describe_keys() against get_keys() for completeness.
• Added an integration test in tests/integration_python/test_main_cli.py covering: unfiltered describe, key-filtered describe, set-value
  rendering, JSON output shape, and unknown-key error.

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.