add screenshots to demos

[jurgenvinju]

Adding screenshots is a matter of using rascal-shell blocks with webapps in them:

```rascal-shell
rascal>import salix::demo::basic::Counter;
ok
rascal>counterWebApp()
ok
```

[jurgenvinju]

To enable preview of the screenshots you should install chrome and chromedriver and provide the right -D options as in .github/workflows/:

-Dwebdriver.chrome.driver=/usr/local/bin/chromedriver -Dwebdriver.chrome.browser=`which chrome`

But you can also leave it to the github action to do this just in time before the publication of the docs. Without the -D stuff the docs simply skip the generation of screenshots and they are not there in the html either as a result.