.gitlab-ci.yml: builds docs on smaller instances

Docs are light-weight and don't need to hog a full s3.large.8 instance,
which is usually busy with Linux builds.

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

.gitlab-ci.yml

@@ -47,6 +47,8 @@ reuse:
 # among others, the "rules" section.
 build-docs:
   extends: [.workspace, .build-docs]
+  variables:
+    CI_ONIRO_INSTANCE_SIZE: s3.large.2
   script:
     - !reference [.workspace, script]
     - !reference [.build-docs, script]