Move README to OLD_README with the intention to remove it.

Created by: garloff

With PR #394, we have moved the docuementation to a new file structure in the doc/ folder. We want to still double-check whether no content from the old README.md has been lost, thus it has not directly been removed -- also we still wait for docusaurus to pick up the docs from our repo here. So for now, rename the README.md file to OLD_README.md with the intention to kill it in the next days. This should ensure that we don't merge content to the outdated README.md in current PRs without noticing -- the PRs should now change the files in the doc/ directory.

Add a new README.md that points to the docusaurus rendered docs.