doc: update meson build instruction (#5899)

The [official build instruction][1] are deprecated on Meson 1.3.1.
These command:

    mkdir -p build && cd build
    meson ..
    ninja

... work but will yield the following warning:

> WARNING: Running the setup command as `meson [options]` instead of `meson setup [options]` is ambiguous and deprecated.

Here's the correct way, according to the [meson documentation][2]:

    mkdir -p build
    meson setup build
    meson compile -C build
    meson install -C build


[1]: https://i3wm.org/docs/hacking-howto.html#_building_i3
[2]: https://mesonbuild.com/Quick-guide.html#compiling-a-meson-project

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