Documentation: Add instructions for network booting

Author: https://github.com/boricj
Commit: https://github.com/SerenityOS/serenity/commit/59396ab1c63
Pull-request: https://github.com/SerenityOS/serenity/pull/5108
Reviewed-by: https://github.com/TheMorc

Documentation/NetworkBoot.md

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+## SerenityOS network booting
+
+By network booting, this guide implies a target computer (physical or virtual) that tries to boot over the network through PXE. The setup presented here is also diskless, as the bootloader and the root file system are supplied over the network.
+
+Note: it is recommended to boot a mainstream operating system through PXE on the target at least once before attempting this, if only to make sure that your setup works.
+
+### General notes
+
+This guide assumes several things:
+- The TFTP server root is `/srv/tftp/`
+- Bootloaders are located inside `/srv/tftp/boot/`
+- SerenityOS artefacts are located inside `/srv/tftp/serenity/`:
+    - The kernel is located at `/srv/tftp/serenity/kernel`
+        - You can find it at `Build/Kernel/Kernel`
+    - The ramdisk is located at `/srv/tftp/serenity/ramdisk`
+        - You can use the QEMU image at `Build/_disk_image` as a ramdisk
+
+### GRUB2
+
+1. Install required packages on the TFTP server
+    - Debian: `apt install grub-pc-bin tftpd-hpa`
+        - Make sure `/srv/tftp/` is owned by the user `tftp`, otherwise the TFTP server won't serve files
+2. Configure the DHCP server with the following options:
+    - Next server IP: `<static IP address of TFTP server>`
+    - Boot filename (for BIOS): `boot/grub/i386-pc/core.0`
+3. Set up a GRUB2 netdir: `grub-mknetdir --net-directory=/srv/tftp -d /usr/lib/grub/i386-pc`
+4. Put your `grub.cfg` configuration file inside `/srv/tftp/boot/grub/`
+5. Place the SerenityOS kernel and ramdisk inside `/srv/tftp/boot/grub/serenity/`
+
+Sample GRUB2 `grub.cfg` configuration file:
+```
+set gfxmode=auto
+insmod all_video
+insmod gfxterm
+terminal_output gfxterm
+
+menuentry 'SerenityOS - netboot diskless graphical mode' {
+        echo 'Loading kernel...'
+        multiboot (tftp)/serenity/kernel root=/dev/ramdisk0
+        echo 'Loading ramdisk...'
+        module (tftp)/serenity/ramdisk
+        echo 'Starting SerenityOS.'
+}
+
+menuentry 'SerenityOS - netboot diskless text mode' {
+        set gfxkeep=text
+        terminal_output console
+        echo 'Loading kernel...'
+        multiboot (tftp)/serenity/kernel root=/dev/ramdisk0 boot_mode=text
+        echo 'Loading ramdisk...'
+        module (tftp)/serenity/ramdisk
+        echo 'Starting SerenityOS.'
+}
+```
+
+### PXELINUX
+
+Warning: PXELINUX cannot set up a framebuffer for Multiboot targets, so you will most likely have no graphics on real hardware.
+
+1. Install required packages on the TFTP server
+    - Debian: `apt install pxelinux tftpd-hpa`
+        - Make sure `/srv/tftp/` is owned by the user `tftp`, otherwise the TFTP server won't serve files
+2. Configure the DHCP server with the following options:
+    - Next server IP: `<static IP address of TFTP server>`
+    - Boot filename (for BIOS): `boot/pxelinux/lpxelinux.0`
+3. Place all the required bootloader modules (located inside `/usr/lib/PXELINUX/` and `/usr/lib/syslinux/modules/bios/` on Debian) inside `/srv/tftp/boot/pxelinux/`, which for the sample configuration file includes:
+    - lpxelinux.0
+    - ldlinux.c32
+    - vesamenu.c32
+    - libcom32.c32
+    - libutil.c32
+    - mboot.c32
+4. Put your `default` configuration file inside `/srv/tftp/boot/pxelinux/pxelinux.cfg/`
+5. Place the SerenityOS kernel and ramdisk inside `/srv/tftp/boot/grub/serenity/`
+
+Sample PXELINUX `default` configuration file:
+
+```
+UI vesamenu.c32
+
+LABEL SerenityOS
+        KERNEL mboot.c32
+        APPEND ../../serenity/kernel root=/dev/ramdisk0 --- ../../serenity/ramdisk
+```
+
+### Troubleshooting
+
+- Issues with DHCP or TFTP usually require sniffing packets on the network to figure out.
+- TFTP is a slow protocol, transferring the QEMU disk image (~ 200 MiB) will take some time. Consider setting up a FTP or HTTP server for faster downloading of SerenityOS artefacts if your bootloader supports it.
+- Remember that SerenityOS has not been extensively tested on physical hardware.
+- Virtual machines can also be booted over the network. Cheat notes for QEMU on Linux, assuming `br0` is already set up:
+
+```
+ip tuntap add tap0 mode tap user <username>
+ip link set tap0 master br0
+ip link set tap0 up
+
+echo 0 > /proc/sys/net/bridge/bridge-nf-call-iptables
+echo 0 > /sys/devices/virtual/net/br0/bridge/multicast_querier
+
+qemu-system-i386 -m 4096 -netdev tap,ifname=tap0,script=no,downscript=no,id=network0 -device e1000,netdev=network0 -boot n -debugcon stdio -s
+```