Installation: install wlroots headers with Meson

Hyprland plugin API requires wlroots headers. Dropping
--skip-subprojects will fix plugins but may break system wlroots.
Instead copy and adjust a part of `make installheaders`.

Example error this tries to fix:

In file included from hy3/src/main.cpp:4:
In file included from /usr/local/include/hyprland/src/Compositor.hpp:7:
In file included from /usr/local/include/hyprland/src/defines.hpp:1:
/usr/local/include/hyprland/src/includes.hpp:36:10: fatal error: 'wlr/backend.h' file not found
   36 | #include <wlr/backend.h>
      |          ^~~~~~~~~~~~~~~

pages/Getting Started/Installation.md

@@ -223,6 +223,9 @@ cd Hyprland
 meson setup build
 meson compile -C build
 meson install -C build --skip-subprojects
+PREFIX=/usr/local
+cd subprojects/wlroots/include && find . -name '*.h*' -print0 | cpio --quiet -0dump ${PREFIX}/include/hyprland/wlroots && cd ../../..
+cd build/subprojects/wlroots/include && find . -name '*.h*' -print0 | cpio --quiet -0dump ${PREFIX}/include/hyprland/wlroots && cd ../../../..
 ```
 
 Refer to [Debugging](../../Contributing-and-Debugging) to see how to build &