Update update-contao.de.md to Contao 5.6

[FlowinBeatz]

Changed Contao 5.5 mentions to Contao 5.6

[zoglo]

We could and should use the SiteParam

https://learn.netlify.app/en/shortcodes/siteparam/
#1670

Sidenote:
You could have used github.dev to stage both file changes in your repository, that way you wouldn't have needed another PR #1679

[FlowinBeatz]

There is no siteparam for bugifx releases and major releases

I used the Interface's edit button. What should I do instead?

[zoglo]

There is no siteparam for bugifx releases and major releases

There is one for 5.6

docs/page/config/_default/params.yml

Line 16 in 06182c6

currentContaoVersion: '5.6'

I used the Interface's edit button. What should I do instead?

It's fine the way you are doing it, I just wanted to mention that you can also use VSCode in the browser by pressing the . button in your fork and then doing multiple changes ;)