Moving toward sustainability in our best practices

[vmassol]

@MichaelHamann

What impact does this change have on:

1. the environmental impact/the resource usage of XWiki?
2. performance (both server- and client-side)?
3. security?
4. usability?
5. privacy?
6. accessibility?

Interestingly, I discussed with @CharpentierLucas about proposing Make sure PR writers have thought about transverse topics