docs/system/target-arm.rst: Add some introductory text

Now we've moved the various bits of per-board documentation into
their own files, the top level document is a little bare. Add
some introductory information, including a note that many
of the board models we support are currently undocumented.

(Most sections of this new text were originally written by me
for the wiki page https://wiki.qemu.org/Documentation/Platforms/ARM)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20200309215818.2021-4-peter.maydell@linaro.org

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