Improve GitHub status message: show # of changed files

[wilzbach]

This is something that should be visible on the CI dashboard as it should be immediately a red flag when the number of changed files is high. To save the space required for this, I changed "Documentation OK" to just "OK".

[CyberShadow]

Showing # of changed files was something I considered back when designing it... I think in the end the number of lines changed is more important and carries at least as much signal for most circumstances.

Removing the "Documentation" word is probably not a good idea. Including it was also a very deliberate decision. Until you learn what exactly "CyberShadow/DAutoTest" means, you don't know what this CI is doing or which CI is responsible for building the docs.