Add CI pipeline to build guide automatically

.gitlab-ci.yml

+stages: [ build ]
+
+job_build:
+  stage: build
+  image: ubuntu:latest
+  script:
+    - apt update
+    - apt install -y --no-install-recommends asciidoctor
+    - gem install asciidoctor-pdf --pre
+    - asciidoctor-pdf guide.adoc
+  artifacts:
+    paths:
+      - "guide.pdf"
+    expire_in: 12 month
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