Documentation: Add more specific instructions on how to use the GN build

Author: https://github.com/ADKaster
Commit: https://github.com/SerenityOS/serenity/commit/e5415f6d86
Pull-request: https://github.com/SerenityOS/serenity/pull/23893
Reviewed-by: https://github.com/nico ✅
Reviewed-by: https://github.com/trflynn89

Documentation/BuildInstructionsLadybird.md

@@ -175,6 +175,15 @@ open -W --stdout $(tty) --stderr $(tty) ./Build/ladybird/bin/Ladybird.app
 open -W --stdout $(tty) --stderr $(tty) ./Build/ladybird/bin/Ladybird.app --args https://ladybird.dev
 ```
 
+### Experimental GN build
+
+There is an experimental GN build for Ladybird. It is not officially supported, but it is kept up to date on a best-effort
+basis by interested contributors. See the [GN build instructions](../Meta/gn/README.md) for more information.
+
+In general, the GN build organizes ninja rules in a more compact way than the CMake build, and it may be faster on some systems.
+GN also allows building host and cross-targets in the same build directory, which is useful for managing dependencies on host tools when
+cross-compiling to other platforms.
+
 ### Debugging with CLion
 
 Ladybird should be built with debug symbols first. In `Meta/CMake/lagom_compile_options.cmake` remove the optimizations by changing `-O2` to `-O0`. For macOS also change the debug option from `-g1` to `-g` so that lldb is happy with the emitted symbols. In linux `-g1` can be changed to `-ggdb3` for maximum debug info.

Meta/gn/README.md

@@ -6,7 +6,35 @@ GN is a metabuild system. It always creates ninja files, but it can create some
 
 This is a good [overview of GN](https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit#slide=id.g119d702868_0_12).
 
-For more information, motivation, philosophy, and inspriation, see the LLVM documentation on its [GN build](https://github.com/llvm/llvm-project/tree/main/llvm/utils/gn#quick-start)
+For more information, motivation, philosophy, and inspiration, see the LLVM documentation on its [GN build](https://github.com/llvm/llvm-project/tree/main/llvm/utils/gn#quick-start)
+
+# Creating a gn build
+
+To create a GN build, you need to have GN installed. You can install it via homebrew on macOS, or via your package manager on Linux.
+On Ubuntu 22.04, the main package repos do not have an up to date enough package for GN, so you will need to build it from source or get a binary from Google.
+
+The easiest way to build GN from source is to use our [Toolchain/BuildGN.sh](../../Toolchain/BuildGN.sh) script, which will
+drop the built binary into the `Toolchain/Local/gn/bin` directory. The instructions for downloading a prebuilt binary from Google are
+[here](https://gn.googlesource.com/gn/+/refs/heads/main#getting-a-binary).
+
+Once you have GN installed, you can create a build directory by running the following commands:
+
+```sh
+gn gen out
+```
+
+`gn gen` creates a ninja build in the `out` directory. You can then build the project with ninja:
+
+```sh
+ninja -C out
+```
+
+If GN or ninja report a bunch of errors, it's likely that you need to create an `args.gn` that points to all the required tools.
+`args.gn` belongs at the root of the build directory, and can be placed there before running gn gen, or modified with
+`gn args <build dir>`. See the section below for a typical `args.gn`.
+
+If you modify `args.gn` outside of `gn args`, be sure to run `gn gen` again to regenerate the ninja files.
+
 
 # Typical gn args
 
@@ -28,3 +56,27 @@ qt_install_libexec="/usr/lib/qt6/libexec/"
 ```
 
 As with any gn project, ``gn args <build dir> --list`` is your best friend.
+
+# Running binaries from the GN build
+
+Targets in the gn build are prefixed by the directory they are declared in. For example, to build the default target
+in the Ladybird/ directory and LibWeb, you would run:
+
+```shell
+ninja -C out Ladybird
+ninja -C out Userland/Libraries/LibWeb
+```
+
+Binaries are placed in the `out/bin` directory, and can be run from there.
+
+```shell
+./out/bin/Ladybird
+# or on macOS
+open -W --stdout $(tty) --stderr $(tty) ./out/bin/Ladybird.app --args https://ladybird.dev
+```
+
+There is also an incomplete target for SerenityOS, which can be tested with:
+
+```shell
+ninja -C out serenity
+```