automates words.md -> index.html with github workflow

[jkoelling]

Already in use in the German translation: https://github.com/TQueV/ (originally added by @lenaschimmel) Based on the original idea with a split index.html file in the Italian translation: https://github.com/harisont/covid-19

The workflow will:

1. checkout master
2. convert md to html with pandoc
3. merge html files by replacing everything between the article tags in the index.html with the content of the words.html
4. commit changes if any were made and push back to master