docs: run `meson setup` explicitly in build instructions
This silences the following warning given on newer version of meson:
WARNING: Running the setup command as meson [options]
instead of
meson setup [options]
is ambiguous and deprecated.
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This silences the following warning given on newer version of meson:
WARNING: Running the setup command as meson [options]
instead of
meson setup [options]
is ambiguous and deprecated.