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

The manifest repository now offers new hidden jobs, that make
customization and reuse easier. Those allow us to remove bulk of the
custom logic, again, reducing everything to mere configuration.

The build-docs and update-docs jobs are entirely replaced by .build-docs
and .aggregate-docs (name change without logic change).

Custom .build job is split into smaller changes to the .workspace and
.bitbake-workspace jobs. Lastly, the variable OHOS_CI_GIT_REPO_PATH
is no longer relative to sources/ but includes it explicitly.

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