gitlab-ci.yml: fix rules:if for build-docs job

There are two separate issues here:

- CI_BRANCH_NAME doesn't exist, we want CI_COMMIT_BRANCH instead.
- The rules engine doesn't handle quotes in the normal sense.
Quotes are only allowed on literals, not variable references.

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