Docs site (#269)

* Set up documentation site using jekyll

* Add workflow for deploying docs

* Ini formatting is hard to read

* Add instructions on how to run docs locally

* Work through docs

* Remove content from README

* Miscellaneous fixes

* Fix artifact upload

docs/README.md

@@ -0,0 +1,14 @@
+# Documentation site
+
+This directory contains the sources for the documentation site published at <https://offen.github.io/docker-volume-backup>.
+
+Assuming you have Ruby and [`bundler`][bundler] installed, you can run the site locally using the following commands:
+
+```
+bundle install
+bundle exec jekyll serve
+```
+
+Note that changes in `_config.yml` require a manual restart to take effect.
+
+[bundler]: https://bundler.io/