Make readme symlinks work on stable branches

!1499 (merged) didn't account for the stable branches, and we now have redirect loops. This MR fixes that by using the symlink_readmes raketask and removing the old setup.

Local test:

# Build needed images
git fetch origin
git checkout -b "symlink-readms-stable-branches" "origin/symlink-readms-stable-branches"
docker pull registry.gitlab.com/gitlab-org/gitlab-docs:base
docker build -t registry.gitlab.com/gitlab-org/gitlab-docs:bootstrap -f dockerfiles/Dockerfile.bootstrap .
docker build -t registry.gitlab.com/gitlab-org/gitlab-docs:builder-onbuild -f dockerfiles/Dockerfile.builder.onbuild .

# Build 13.9 image, new raketask method
git checkout 13.9
docker build --build-arg NANOC_ENV=production --build-arg CI_COMMIT_REF_NAME=13.9 -t registry.gitlab.com/gitlab-org/gitlab-docs:13.9 -f Dockerfile.13.9 .
# Test it and visit http://0.0.0.0:4000/13.9/ee/install/README.html
docker run -it --rm -p 4000:4000 registry.gitlab.com/gitlab-org/gitlab-docs:13.9

# Build 13.8 image, old symlink method
git checkout 13.8
docker build --build-arg NANOC_ENV=production --build-arg CI_COMMIT_REF_NAME=13.8 -t registry.gitlab.com/gitlab-org/gitlab-docs:13.8 -f Dockerfile.13.8 .
# Test it and visit http://0.0.0.0:4000/13.8/ee/install/README.html
docker run -it --rm -p 4000:4000 registry.gitlab.com/gitlab-org/gitlab-docs:13.8