Add a milestone for issues related to the rewrite

[FixItKit]

Checklist

• [X] I made sure that there are no existing issues - open or closed - which I could contribute my information to.
• [X] I have read the FAQ and my problem isn't listed.
• [X] I'm aware that this is a request for NewPipe itself and that requests for adding a new service need to be made at NewPipeExtractor.
• [X] I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
• [X] This issue contains only one feature request.
• [X] I have read and understood the contribution guidelines.

Feature description

There should be a new milestone on GitHub for issues related to the rewrite.

Why do you want this feature?

This will make it easier to track the progress of the rewrite.

Additional information

You could also use milestones to track the progress on regular updates.

[ghost]

On which branch is the rewrite made?

[TobiGr]

We have an internal project which I now made public: https://github.com/orgs/TeamNewPipe/projects/18

The rewrite is happening on the refactor branch. We should add this info to the README...