[core] Improve the feedback loop from developers

[oliviertassinari]

This pull request started out of my frustration of having developers send emails to get support on the Pro plan. This feels inefficient because:

1. We split the place where we collect user's pain into two different locations. It makes it harder to consolidate the problems, note a great product feedback loop.
   Instead, by moving the questions to GitHub, we can for instance: link duplicates, link PRs that are fixing the root causes. A note on StackOverflow. This is a pain we have with it too, the main difference is that the community is answering these support questions. It's a tradeoff we have made to leverage the gamification they have in place to incentive contributions. With the Pro plan, it's us that have to answer, I doubt that the community will have the incentive to contribute much.
2. On the MIT side, each member asking a question and us answering is adding value to the whole user base. We should be able to reproduce this network effect with our paid customers. An instant self-serving experience is often better than waiting 2 days to get an answer.

But while I was working on it, I realized that GitHub had a neat new feature in beta: Issue forms. I went on leveraging it.

You can preview the exact impacts of these changes at /~https://github.com/mui-org/test-issue-template/issues/new/choose. For instance:

The aim is to backport these changes into @mui-org/core if the PR is merged.

[mbrookes]

I have only suggested changes once for duplicate strings.

updated