Added documentation website content, and a workflow to publish it to GitHub Pages

* Copied, rearranged and modified the contents of the old site directory for Jekyll static site generator, into the website directory
* Added some Jekyll-specific things for looks, including:
  * _config.yaml, Gemfile.* and 404.html files
  * _data, _layouts and assets directories and their content
* Added .github/workflows/github-pages-publish.yml workflow

website/_data/topmenu.yaml

@@ -0,0 +1,15 @@
+menuitems:
+  - title: Home
+    url: /
+  - title: Overview
+    url: /overview
+  - title: Concepts
+    url: /concepts
+  - title: Install
+    url: /install
+  - title: Kubernetes
+    url: /kubernetes
+  - title: Operational Guide
+    url: /operational-guide
+  - title: Tasks
+    url: /tasks