tf-ci-users-guide: introduce job to trigger readthedocs build

Introduce job that for every merge-event on the TF Open CI Users Guide
documentation repository [1], a build on 'readthedocs' is triggered,
updating documentation [2] based on the introduced change.

[1] https://git.trustedfirmware.org/ci/tf-ci-users-guide.git/
[2] https://tf-ci-users-guide.readthedocs.io/en/latest/

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I16e2609f5d03edf43ed83d3732a46f3ec1ea099c
1 file changed