[ci] rearrange jobs

.ci_base.yml

-# base for jobs, all run the same script
-.common:
-  script: ./.ci_run_tests
-
 # base for merge requests that affect the core/tests
 .testbeforemerge:
-  only:
-    refs:
-      - merge_requests
-
-.ubuntu18.04 gcc-7:
-  extends: .common
-  variables:
-    CI_COMPILER: gcc
-    CI_COMPILER_VERSION: 7
-
-.ubuntu18.04 gcc-8:
-  extends: .common
-  variables:
-    CI_COMPILER: gcc
-    CI_COMPILER_VERSION: 8
-
-.ubuntu18.04 clang-5:
-  extends: .common
-  variables:
-    CI_COMPILER: clang
-    CI_COMPILER_VERSION: 5
+  script: ./.ci_run_tests
+  except:
+    changes:
+      - "*.md"
+      - doc/*
 
-.ubuntu18.04 clang-6:
-  extends: .common
-  variables:
-    CI_COMPILER: clang
-    CI_COMPILER_VERSION: 6
+# build pipeline for docu
+builddoc:
+  script: ./.ci_build_doc
+  only:
+    changes:
+      - doc/*
 
-.ubuntu18.04 clang-7:
-  extends: .common
-  variables:
-    CI_COMPILER: clang
-    CI_COMPILER_VERSION: 7
+# noop pipeline for changes to markdown files
+markdownedits:
+  script: echo "Only markdown files edited. Skipping pipelines."
+  only:
+    changes:
+      - "*.md"

.gitlab-ci.yml

-include: .ci_base.yml
+include:
+  - template: 'Workflows/MergeRequest-Pipelines.gitlab-ci.yml'
+  - local: .ci_base.yml
 
 # using OpenCascade 7.3
 ubuntu18.04 gcc-7 occ7.3:
-  extends:
-    - .ubuntu18.04 gcc-7
-    - .testbeforemerge
   image: git.iws.uni-stuttgart.de:4567/tools/frackit/ubuntu18.04_occ7.3:latest
+  extends: .testbeforemerge
+  variables:
+    CI_COMPILER: gcc
+    CI_COMPILER_VERSION: 7
 
 ubuntu18.04 gcc-8 occ7.3:
-  extends:
-    - .ubuntu18.04 gcc-8
-    - .testbeforemerge
   image: git.iws.uni-stuttgart.de:4567/tools/frackit/ubuntu18.04_occ7.3:latest
+  extends: .testbeforemerge
+  variables:
+    CI_COMPILER: gcc
+    CI_COMPILER_VERSION: 8
+
+ubuntu18.04 clang-6 occ7.3:
+  image: git.iws.uni-stuttgart.de:4567/tools/frackit/ubuntu18.04_occ7.3:latest
+  extends: .testbeforemerge
+  variables:
+    CI_COMPILER: clang
+    CI_COMPILER_VERSION: 6