[ENH] wishlists in the repository

I think it would be great to curate wishlists of concrete algorithms or objects in the repository, so the process of suggesting additions and picking them for implementation is streamlined.

Why to do this:

• we receive a lot of requests, many of which are good, but some of which are out of scope (e.g., not a time series algorithm). This way, core devs can review feature requests, approve/reject, and even prioritize them.
• it is easier to add items in bulk than it would be in issues. We have some issues that requests many items, and many issues that only request some or just one.
• there are a lot of contributors who would like something to work on, but we are running out of good first issues. Having centralized wishlists for objects to support makes it easier for them to pick them up
• this counts adding to the wishlist as a proper contribution (which it should be), by mapping on repository contribution record and statistics

I suggest to add:

• a wishlist folder in root
• a md document for every scitype currently present in sktime, e.g., classifier, forecaster
• a README summary
• instructions at the top of each md, saying e.g.,:
  • when adding an item to the wishlist, an issue should also be added too
  • basic metadata: name of object requested, optimally a link to paper or another code location (in case of interfacing)
  • reference to the issue - one per object requested

Creating the first structure and populating it with the content of current wishlists (search "wishlist" in issue tracker) - some of which are not well curated since issues get drowned in more issues - would be a good start!

One wishlist per contributor please, e.g., classifier, detector, forecaster, etc. For a list of scitypes, go to sktime.registry._base_classes, and look at the names of classes (ignore object and estimator).