Invert symlink and content for the README file

GitLab will display the content of the README file when browsing the repository. But in case the file is a symlink, it will display the path pointed by the symlink. There is a 6 year old issue about this: https://gitlab.com/gitlab-org/gitlab/-/issues/15093

We can workaround the issue by having the content at the root of the repository and a symlink to this file in the docs/ directory.