Use the 'ee/' path to build the Nanoc site

We can do this now that we have merged CE/EE, the variable
is no longer needed.

The following merge requests fixed a bug we had in the docs
pipeline, but introduced another:

- https://gitlab.com/gitlab-org/gitlab-docs/-/merge_requests/731
- https://gitlab.com/gitlab-org/gitlab-docs/-/merge_requests/734

Namely, forks cannot use the project-set $DOCS_GITLAB_REPO_SUFFIX
variable, so they will fail at the internal links test. For example
https://gitlab.com/briankabiro/gitlab-ee/-/jobs/460294523.

Nanoc would create the content under 'public/doc/', whereas it should
be 'public/ee/', since we have some hardcoded links set in
https://gitlab.com/gitlab-org/gitlab-docs/-/blob/887850752fc0e72856da6632db132f005ba77f16/content/index.erb#L44-63

.gitlab/ci/docs.gitlab-ci.yml

@@ -51,7 +51,9 @@ docs lint:
   script:
     - scripts/lint-doc.sh
     # Prepare docs for build
-    - mv doc/ /tmp/gitlab-docs/content/$DOCS_GITLAB_REPO_SUFFIX
+    # The path must be 'ee/' because we have hardcoded links relying on it
+    # https://gitlab.com/gitlab-org/gitlab-docs/-/blob/887850752fc0e72856da6632db132f005ba77f16/content/index.erb#L44-63
+    - mv doc/ /tmp/gitlab-docs/content/ee
     - cd /tmp/gitlab-docs
     # Build HTML from Markdown
     - bundle exec nanoc