Add a "Peertube instances" domain name example & validation

@Maintainer: Could you please check if my assumption is correct or which kind of domain format is expected?

Looking at the code here: https://git.drupalcode.org/issue/peertube-3362469/-/blob/3362469-add-dom...

it seems the protocol might expected?
Would it then perhaps make sense to validate the entered URL to save users from issues with unexpected URL formats?

After looking at the code and testing around, it's not clear, that the URL needs to include the scheme part, without trailing slash!
So a valid example is: https://www.mypeertubeurl.com!

Here we go finally. I improved the documentation and added validateForm() which checks the URLs for the expected format. Tested everything manually and it works perfectly fine.

Please review.

LGTM!

Great UX addition! 🙂👍

Sorry for the delay, looks great.
Thank you for the merge request!

Automatically closed - issue fixed for 2 weeks with no activity.