docs: add hotlinks to about preface text

Make it easier to navigate the documentation.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230124180127.1881110-20-alex.bennee@linaro.org>

docs/about/index.rst

@@ -5,19 +5,19 @@ About QEMU
 QEMU is a generic and open source machine emulator and virtualizer.
 
 QEMU can be used in several different ways. The most common is for
-"system emulation", where it provides a virtual model of an
+:ref:`System Emulation`, where it provides a virtual model of an
 entire machine (CPU, memory and emulated devices) to run a guest OS.
-In this mode the CPU may be fully emulated, or it may work with
-a hypervisor such as KVM, Xen, Hax or Hypervisor.Framework to
-allow the guest to run directly on the host CPU.
+In this mode the CPU may be fully emulated, or it may work with a
+hypervisor such as KVM, Xen, Hax or Hypervisor.Framework to allow the
+guest to run directly on the host CPU.
 
-The second supported way to use QEMU is "user mode emulation",
+The second supported way to use QEMU is :ref:`User Mode Emulation`,
 where QEMU can launch processes compiled for one CPU on another CPU.
 In this mode the CPU is always emulated.
 
-QEMU also provides a number of standalone commandline utilities,
-such as the ``qemu-img`` disk image utility that allows you to create,
-convert and modify disk images.
+QEMU also provides a number of standalone :ref:`command line
+utilities<Tools>`, such as the ``qemu-img`` disk image utility that
+allows you to create, convert and modify disk images.
 
 .. toctree::
    :maxdepth: 2

docs/system/index.rst

@@ -1,3 +1,5 @@
+.. _System Emulation:
+
 ----------------
 System Emulation
 ----------------

docs/tools/index.rst

@@ -1,3 +1,5 @@
+.. _Tools:
+
 -----
 Tools
 -----

docs/user/index.rst

@@ -1,3 +1,5 @@
+.. _User Mode Emulation:
+
 -------------------
 User Mode Emulation
 -------------------