docs: stop tracking placeholder documentation

The placeholder documentation introduces a couple of problems:
- it complicates the contributor-experience (forces the CI to run
N times before the contributor finds out that they need to call an .sh
script and include certain files from docs/)
- it forces CLI related pull requests for tools like kubeadm and kubectl
to require top level approval from docs/OWNERS as such PRs still need
to touch the .generated_docs file

Stop tracking the placeholder documentation by applying the
following actions:
- remove the utility set-placeholder-gen-docs()
- make verify-generated-docs.sh only generate in a temporary folder
and not match .generated_docs
- mark generate-docs.sh as an alias for update-generated-docs.sh
- remove all current placeholder files in docs folders admin, man,
user-guide, yaml
- ignore the above folders and .generated_docs in a .gitignore file

hack/lib/util.sh

@@ -228,21 +228,6 @@ kube::util::gen-docs() {
   popd > /dev/null
 }
 
-# Puts a placeholder for every generated doc. This makes the link checker work.
-kube::util::set-placeholder-gen-docs() {
-  local list_file="${KUBE_ROOT}/docs/.generated_docs"
-  if [[ -e "${list_file}" ]]; then
-    # remove all of the old docs; we don't want to check them in.
-    while read file; do
-      if [[ "${list_file}" != "${KUBE_ROOT}/${file}" ]]; then
-        cp "${KUBE_ROOT}/hack/autogenerated_placeholder.txt" "${KUBE_ROOT}/${file}"
-      fi
-    done <"${list_file}"
-    # The docs/.generated_docs file lists itself, so we don't need to explicitly
-    # delete it.
-  fi
-}
-
 # Removes previously generated docs-- we don't want to check them in. $KUBE_ROOT
 # must be set.
 kube::util::remove-gen-docs() {