Amend documentation to reflect that `meson install` is not run

Amend the documentation to reflect the fact that we don't actually run meson install but just re-implement it. This affects two Meson features: `install_rpath` support, and running scripts registered with `meson.add_install_script()`. Both things have no effect when building a package with `meson-python`.

Inspect the project documentation describing installation and package building. Update it to explain that meson-python re-implements installation rather than running `meson install`, including that `install_rpath` and scripts registered with `meson.add_install_script()` have no effect. Done means both limitations are accurately documented.

Written by the indexing model from the issue text.