ci: allow forks to have different names

When the fork of the "docs" repository is called something else, we need to
refer to it by the real name to successfully perform the replacement for merge
request pipelines.

Use some bash and CI_MERGE_REQUEST_SOURCE_PROJECT_PATH to find the bare name of
the project. This works because the variable is re-interpolated by bash during
job execution.

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