docs: Add some actual About text to about/index.rst

Add some text to About to act as a brief introduction to the QEMU
manual and to make the about page a bit less of an abrupt start to
it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210705095547.15790-6-peter.maydell@linaro.org

docs/about/index.rst

@@ -1,6 +1,23 @@
 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
+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.
+
+The second supported way to use QEMU is "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.
+
 .. toctree::
    :maxdepth: 2