Show PyPI installation path for packages available on PyPI

[csadorf]

Some RAPIDS packages (cuDF, cuML, and dask-cuDF) are now available on PyPI, but the install selector always shows installation commands with --extra-index-url pointing to NVIDIA's package index.

Expected behavior:
When users select only packages available on PyPI, the install command should install directly from PyPI without the --extra-index-url flag, making installation simpler and leveraging PyPI's infrastructure.

Acceptance criteria:

• Install selector detects when only PyPI-available packages are selected
• For PyPI-only selections, omit --extra-index-url from the install command
• Update install notes to indicate PyPI availability
• Maintain existing behavior for packages requiring NVIDIA's index