Merge branch 'gitlab-pages' into 'master'

Use gitlab-pages

This use GitLab Pages to serve the `doc-gitlab-com`.

It should be available under: https://gitlab-com.gitlab.io/doc-gitlab-com.

See merge request !25

.gitlab-ci.yml

+image: registry.gitlab.com/gitlab-org/gitlab-build-images:doc-gitlab-com
+
+variables:
+  SITE_PATH: "public/"
+  PROGRESS: "1"
+
+pages:
+  script:
+  - ruby -v
+  - ruby generate.rb
+  # Symlink all README.html to index.html for backwards compatibility. The site
+  # was deployed in a VPS and served by NGINX.
+  - find public -type d -not -path public | xargs -I{} ln -s README.html "{}/index.html"
+  artifacts:
+    paths:
+    - public/
+  only:
+    - master