gitlab-ci.yml: use new jobs from the manifest repo

The .workspace and .build-docs allow simplification of the two similar
jobs present into this repository. The .workspace job completely
replaces the local copy, with a single configuration variable to enable
testing of incoming changes.

The build job becomes build-docs and extends both the .workspace job and
the .build-docs job, reusing both the scripts to perform everything that
was formerly specified explicitly.

The deploy job now extends the .workspace job but is otherwise
unchanged.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>