Add special goto def/docs support in doom! blocks

- Pressing gd on a module in your doom! block will now browse that
  module's directory.
- Pressing K on a module will jump to that module's documentation, if any.
- Pressing K on a module flag will jump to that flag's description
  within that module's documenation.
- This is now explained in init.example.el

Closes #2249

init.example.el

@@ -1,12 +1,18 @@
 ;;; init.el -*- lexical-binding: t; -*-
 
-;; Copy this file to ~/.doom.d/init.el or ~/.config/doom/init.el ('doom install'
-;; will do this for you). The `doom!' block below controls what modules are
-;; enabled and in what order they will be loaded. Remember to run 'doom refresh'
-;; after modifying it.
+;; This file controls what Doom modules are enabled and what order they load in.
+;; Remember to run 'doom sync' after modifying it!
+
+;; NOTE Press 'SPC h d h' (or 'C-h d h' for non-vim users) to access Doom's
+;;      documentation. There you'll find information about all of Doom's modules
+;;      and what flags they support.
+
+;; NOTE Move your cursor over a module's name (or its flags) and press 'K' (or
+;;      'C-c g k' for non-vim users) to view its documentation. This works on
+;;      flags as well (those symbols that start with a plus).
 ;;
-;; More information about these modules (and what flags they support) can be
-;; found in modules/README.org.
+;;      Alternatively, press 'gd' (or 'C-c g d') on a module to browse its
+;;      directory (for easy access to its source code).
 
 (doom! :input
        ;;chinese