docs: provide example_init.lua #33524

Problem:
There are some "boilerplate" steps for new users. Although we are
constantly improving defaults and lifting patterns into core, users
eventually want to know how to start their own config, add plugins, etc.

Solution:
Add `runtime/example_init.lua` and refer to it from docs.

runtime/tutor/en/vim-01-beginner.tutor

@@ -967,6 +967,10 @@ NOTE: Completion works for many commands. It is especially useful for `:help`{vi
  6. While in command mode, press `<C-d>`{normal} to see possible completions.
     Press `<Tab>`{normal} to use the completion menu and select a match.
 
+# What's next?
+
+Run `:help nvim-quickstart`{vim} to get started with configuring or extending Nvim.
+
 # CONCLUSION
 
 This was intended to give a brief overview of the Neovim editor, just enough to