chore(all): Specify plugin requirements, clean up README files

[vbuberen]

Description

This PR focuses on improving README files for every plugin.

1. Explicitly added Requirements section to every plugin to cause less confusion among users, who don't see updated requirements in Changelogs.
2. Fixed the Flutter Favorites image as for some plugins is had wrong path specified and didn't render on both Github and pub.dev website. Here is an example:

Image is now placed in assets folder to simplify website folder removal when we agree to do so.

3. Removed those Flutter Community banners from the top of every plugin as they were stealing a lot of space and didn't really bring any value from my point of view, because in any case Flutter Community is mentioned on pub.dev as publisher, plugins are placed in the Flutter Community Github org, etc.

Here is a sneak peak on updated README with fixed image and removed banner: