Merge !1182

Add gitlab-ci.yml file for an initial CI setup

See merge request monero-project/monero-site!1182

gitlab-ci.yml

+image: ruby:2.5
+stages:
+  - build
+
+before_script:
+  - gem install bundler
+
+build_site:
+  stage: build
+  only:
+    - master
+  script:
+    - bundle install
+    - bundle exec jekyll build