[v0.22] add checking for invalid characters that will break .mdx generation in docs to hack/schema/main.go; fix sync-config-schema.yaml vcluster-config syncing (#2348)

[v0.22] add checking for invalid characters that will break .mdx generation in docs to hack/schema/main.go; fix sync-config-schema.yaml vcluster-config syncing (#2348) #4157