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.

test/functional/plugin/tutor_spec.lua

@@ -131,7 +131,7 @@ describe(':Tutor', function()
       {0:  }University. E-mail: {2:bware@mines.colorado.edu}.                                  |
     ]]
 
-    feed(':969<CR>zt')
+    feed(':973<CR>zt')
     screen:expect(expected)
   end)
 end)