Move contents of `Wiki` to `README.md` and empty out `Wiki` page.

[JooHyukKim]

As a user's perspective, it is better to have less place to expose informations.

Sadly I can't make a PR on Wiki itself 😅

[Marcono1234]

It looks like there was also some (accidental?) vandalism in the past. For example https://github.com/FasterXML/jackson-docs/wiki/JacksonOnAndroid currently just contains "ii" as text. But at least it would be possible to restore the content.

The wiki seems to be not editable at the moment, which is good to avoid vandalism but prevents correcting incorrect information. So having the content as part of the repository would also make it easier to contribute changes.