Out documentation directory is doc/ and not docs/.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

readthedocs.org allows to automatically generate and publish the HTML
documentation for the U-Boot project.

Add a file controlling building https://u-boot.readthedocs.io/

.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>