Now we have some rST format docs in the docs/specs/ manual, we should
actually build and install it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-id: 20190610152444.20859-3-peter.maydell@linaro.org