Documentation: Move Ubuntu/Debian specific build instructions

Ubuntu/Debian specific instructions should live under the
Ubuntu/Debian header

Documentation/BuildInstructions.md

@@ -10,7 +10,17 @@ Ensure your [QEMU](https://www.qemu.org/) version is >= 5 with `qemu-system-i386
 
 Ensure your gcc version is >= 10 with `gcc --version`. Otherwise, install it.
 
-On Ubuntu it's in the repositories of 20.04 (Focal) and later - add the `ubuntu-toolchain-r/test` PPA if you're running an older version:
+Make sure you have all the dependencies installed (`ninja` is optional, but is faster in practice):
+
+#### Debian / Ubuntu
+
+```console
+sudo apt install build-essential cmake curl libmpfr-dev libmpc-dev libgmp-dev e2fsprogs ninja-build qemu-system-i386 qemu-utils ccache rsync
+```
+
+##### GCC 10
+
+On Ubuntu gcc-10 is available in the repositories of 20.04 (Focal) and later - add the `ubuntu-toolchain-r/test` PPA if you're running an older version:
 
 ```console
 sudo add-apt-repository ppa:ubuntu-toolchain-r/test
@@ -37,12 +47,13 @@ sudo sed -i '$d' /etc/apt/sources.list
 sudo apt update
 ```
 
-Make sure you have all the dependencies installed (`ninja` is optional, but is faster in practice):
+##### QEMU 5
 
-#### Debian / Ubuntu
+QUEMU version 5 is available in Ubuntu 20.10. For earlier versions, you can build it using the `Toolchain/BuildQemu.sh` script.
+You may need the gtk+ 3.0 dev package:
 
 ```console
-sudo apt install build-essential cmake curl libmpfr-dev libmpc-dev libgmp-dev e2fsprogs ninja-build qemu-system-i386 qemu-utils ccache rsync
+sudo apt install libgtk-3-dev
 ```
 
 #### Fedora