docs/system: convert Texinfo documentation to rST

Apart from targets.rst, which was written by hand, this is an automated
conversion obtained with the following command:

  makeinfo --force -o - --docbook \
    -D 'qemu_system_x86 QEMU_SYSTEM_X86_MACRO' \
    -D 'qemu_system     QEMU_SYSTEM_MACRO' \
    $texi | pandoc -f docbook -t rst+smart | perl -e '
      $/=undef;
      $_ = <>;
      s/^-  − /-  /gm;
      s/QEMU_SYSTEM_MACRO/|qemu_system|/g;
      s/QEMU_SYSTEM_X86_MACRO/|qemu_system_x86|/g;
      s/(?=::\n\n +\|qemu)/.. parsed-literal/g;
      s/:\n\n::$/::/gm;
      print' > $rst

In addition, the following changes were made manually:

- target-i386.rst and target-mips.rst: replace CPU model documentation with
  an include directive

- monitor.rst: replace the command section with a comment

- images.rst: add toctree

- target-arm.rst: Replace use of :math: (which Sphinx complains
  about) with :sup:, and hide it behind |I2C| and |I2C| substitutions.

Content that is not @included remains exclusive to qemu-doc.texi.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200228153619.9906-20-peter.maydell@linaro.org
Message-id: 20200226113034.6741-19-pbonzini@redhat.com
[PMM: Fixed target-arm.rst use of :math:; remove out of date
 note about images.rst from commit message; fixed expansion
 of |qemu_system_x86|; use parsed-literal in invocation.rst
 when we want to use |qemu_system_x86|; fix incorrect subsection
 level for "OS requirements" in target-i386.rst; fix incorrect
 syntax for making links to other sections of the manual]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/linuxboot.rst

@@ -0,0 +1,30 @@
+.. _direct_005flinux_005fboot:
+
+Direct Linux Boot
+-----------------
+
+This section explains how to launch a Linux kernel inside QEMU without
+having to make a full bootable image. It is very useful for fast Linux
+kernel testing.
+
+The syntax is:
+
+.. parsed-literal::
+
+   |qemu_system| -kernel bzImage -hda rootdisk.img -append "root=/dev/hda"
+
+Use ``-kernel`` to provide the Linux kernel image and ``-append`` to
+give the kernel command line arguments. The ``-initrd`` option can be
+used to provide an INITRD image.
+
+If you do not need graphical output, you can disable it and redirect the
+virtual serial port and the QEMU monitor to the console with the
+``-nographic`` option. The typical command line is:
+
+.. parsed-literal::
+
+   |qemu_system| -kernel bzImage -hda rootdisk.img \
+                    -append "root=/dev/hda console=ttyS0" -nographic
+
+Use Ctrl-a c to switch between the serial console and the monitor (see
+:ref:`pcsys_005fkeys`).