Clean README in post release job as well.
# What does this PR do?

The README cleanup (removing the main in the links to the doc) hasn't been done in the past releases because we do them on branches (so it has been done, but not on main). This PR redoes the cleanup in the `post-release` job (which is always done on main) and updates the instruction in the setup (no need to run `make post-patch` anymore but the post-release job will change the README so `make fix-copies` is necessary).