docs: Update build commands
docs: Update build commands

Updated build instructions for .NET on Haiku:
- For `./build-rootfs.sh`, changed the `--rootfs-dir` switch to `--rootfsdir`. Only the second one works with the current script.
- Added important targets mentioned in dotnet#55803
- Added instructions to build and run the PAL tests.
jessicah/jessicah.github.io,jessicah/jessicah.github.io