Documentation: Reorganize the build documentation

Documentation/BuildInstructionsMacOS.md

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+# Setting up a development environment on macOS
+
+# Prerequisites
+
+This installation guide assumes that you have Homebrew, Xcode and `xcode-tools` installed.
+
+Make sure you also have all the following dependencies installed:
+
+```console
+# core
+brew install coreutils qemu bash gcc@10 ninja cmake ccache rsync
+
+# (option 1) fuse + ext2
+brew install e2fsprogs m4 autoconf automake libtool
+brew install --cask osxfuse
+Toolchain/BuildFuseExt2.sh
+
+# (option 2) genext2fs
+brew install genext2fs
+```
+
+Notes:
+
+- Installing osxfuse for the first time requires enabling its system extension in System Preferences and then restarting
+  your machine. The output from installing osxfuse with brew says this, but it's easy to miss.
+  
+## Hardware acceleration on macOS Big Sur
+
+If you are on macOS Big Sur, you will need to manually enable QEMU's hardware acceleration before running Serenity, by
+creating a new file called `entitlements.xml` in the `Build/` folder, with the content below, and then running this
+command:
+
+`codesign -s - --entitlements entitlements.xml --force /usr/local/bin/qemu-system-x86_64`
+
+<details>
+<summary>Content for 'entitlements.xml'.</summary>
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
+	"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>com.apple.security.hypervisor</key>
+    <true/>
+</dict>
+</plist>
+```
+
+</details>
+