[docs] Mention lsp-start-plain.el on troubleshooting

Author: ericdallo

docs/page/troubleshooting.md

@@ -29,3 +29,11 @@ If you manage your Emacs packages with the built-in `package.el`, we recommend t
 1. Delete your LSP-related packages
 2. Restart Emacs
 3. Install the new versions of the packages.
+
+#### Check if any package or wrong config could be causing the issue
+
+Try to repro the issue with our [lsp-start-plain.el](https://github.com/emacs-lsp/lsp-mode/blob/master/scripts/lsp-start-plain.el) file, which has just the necessary config/packages for lsp-mode work.
+
+```bash
+emacs -q -l lsp-start-plain.el
+```