Use new docker-based docs tool (elastic#41985)

The old perl-based docs tool was previously deprecated and now no longer
works.

src/docs/docs_repo.js

@@ -22,7 +22,7 @@ import { resolve } from 'path';
 const kibanaDir = resolve(__dirname, '..', '..');
 
 export function buildDocsScript(cmd) {
-  return resolve(process.cwd(), cmd.docrepo, 'build_docs.pl');
+  return resolve(process.cwd(), cmd.docrepo, 'build_docs');
 }
 
 export function buildDocsArgs(cmd) {