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

Author: neolit123

docs/.generated_docs

@@ -0,0 +1,5 @@
+.generated_docs
+admin/
+man/
+user-guide/
+yaml/