Move all documentation images into repo at docs/images/

[mariobehling]

Images on the Readme.md and possibly other documentation files have images loading from external sources. This makes us dependent on external source being up and running. We don't want that.

Therefore please move all images in the documentation files including badges from app stores to /docs/images and update the files.

[ShridharGoel]

There is already a PR for this: #1668