Add basic gitpod config

Trying out gitpod.io to see if we can use it as a simple way to get people
a working dev environment to work on the documentation.

.gitpod.yml

@@ -0,0 +1,9 @@
+tasks:
+  - init: |
+      pip3 install -r docs/doc-requirements.txt
+      pip3 install -e.
+    command: |
+      cd docs && make html &&  cd build/html && python3 -m http.server 8080
+    name: Sphinx preview
+ports:
+- port: 8080