docs/conf.py: Don't include rST sources in HTML build

Sphinx defaults to including all the rST source files
in the HTML build and making each HTML page link to the
source file. Disable that.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190305172139.32662-7-peter.maydell@linaro.org
Message-id: 20190228145624.24885-7-peter.maydell@linaro.org