Update README troubleshooting and beta scope

[teor2345]

Motivation

1. We shared some troubleshooting tips with external users, so we want to add them to the README
2. macOS can report crashes in the Zebra tests, which is confusing for users
3. The "current features" section is outdated and overlaps with the "beta" section
4. The summary of missing features in the beta section is confusing and incomplete

This is unscheduled but expected beta-related work in sprint 22.

Closes #2990.

Solution

1. Add troubleshooting tips to the README
2. Document that macOS can report crashes in the Zebra tests, but they are intentional
3. Delete the "current features" section
4. Update the summary in the beta section from the detailed list

Review

Anyone can review this low-priority PR.

Reviewer Checklist

• Documentation makes sense
• Do we want to keep anything from the "current features" section?

[mpguerra]

This looks great!

I am struggling with what to do about the "Current features" section. If everything that we explicitly covered in this section is still covered by our broad explanation about what zebra is and does then I'm happy to remove.

If we get questions or issues opened in future because users are confused about why zebra is or isn't doing something then we should consider adding it back in