Clarify documentation and add example values

Problem/Motivation

The README currently says:

Visit /admin/config/media/peertube and add the domains of your Peertube instances you want to use.
Configure the oEmbed Providers module at /admin/config/media/oembed-providers/custom-providers and allow the Peertube provider.

/admin/config/media/peertube currently says:

Contribute all the domain names of the PeerTube instances you want to use. For now, only the last one will be supported due to a core issue.

But it's unclear, which format to use.

For example is it:

• myexamplevideoplatform.com
• https://myexamplevideoplatform.com

Would be great, if examples could be added to the README and code to clarify that.
We're currently having issues connecting a custom peertube instance and are unsure if a value might be wrong?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes