[libc++][doc] Updates module build instructions. (#89413)

CMake has landed experimental support for using the Standard modules.
This will be part of the CMake 3.30 release. This updates the build
instructions to use modules with CMake.

The changes have been tested locally.

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Co-authored-by: Will Hawkins <whh8b@obs.cr>
GitOrigin-RevId: 033453a9ad2a62914358747b5beb347482d3fdbd
1 file changed