multi-lingual CWL guide

- ~~[Fix inter links issue by converting the markdown to RST](https://github.com/common-workflow-language/user_guide/issues/368)~~
- [x] configure sphinx to build a multi-lingual site https://github.com/common-workflow-language/user_guide/pull/442
- [x] build English version at `en/`
- [ ] add [redirect static pages](https://github.com/common-workflow-language/common-workflow-language.github.io/blob/master/rnaseq-training/05-scatter/index.html) at the old paths pointing to the new paths
- [ ] the key is ` `

Later:
- [ ] once the above is done, add a language switcher to the header and/or sidebar
- [ ] add a "translate this page" link to the sidebar
- [ ] Add a credit for the translation platform hosting. Maybe rename the "LICENSE" page to be about the User Guide itself and include credits & contributing information there?
- [ ] bonus, can we auto-guess the language based upon the `Accept-Language` header, for those who have it set?

~~See https://github.com/swcarpentry/styles/issues/210
especially this list of Jekyll plugins for multilingualism: https://github.com/Anthony-Gaudino/jekyll-multiple-languages-plugin#10-other-language-plugins~~