Add package management to tutorial and startup screen

* etc/tutorials/TUTORIAL: Add section about package installation.
* lisp/startup.el (fancy-startup-tail): Add button for
package-list-packages.

This was discussed in:
https://lists.gnu.org/r/emacs-devel/2021-12/msg00079.html

etc/tutorials/TUTORIAL

@@ -1099,6 +1099,20 @@ manual in the node called "Dired".
 The manual also describes many other Emacs features.
 
 
+* INSTALLING PACKAGES
+---------------------
+
+Emacs has a rich set of packages written by the community, which
+extends Emacs' capabilities.  These features include support for new
+languages, additional themes, plugins for integrating with external
+applications and much much more.
+
+To see a list of all available packages, type M-x list-packages.  In
+this mode, you can install or uninstall packages, as well as read
+packages' descriptions.  For more information about package
+management, consult the manual.
+
+
 * CONCLUSION
 ------------
 

lisp/startup.el

@@ -1840,18 +1840,22 @@ a face or button specification."
   (unless concise
     (fancy-splash-insert
      :face 'variable-pitch
-     "\nTo start...     "
+     "\nTo start...\t"
      :link `("Open a File"
 	     ,(lambda (_button) (call-interactively 'find-file))
 	     "Specify a new file's name, to edit the file")
-     "     "
+     "\t\t"
      :link `("Open Home Directory"
 	     ,(lambda (_button) (dired "~"))
 	     "Open your home directory, to operate on its files")
-     "     "
+     "\n\t"
      :link `("Customize Startup"
 	     ,(lambda (_button) (customize-group 'initialization))
 	     "Change initialization settings including this screen")
+     "\t"
+     :link `("Explore Packages"
+	     ,(lambda (_button) (call-interactively 'package-list-packages))
+	     "Explore, install and remove Emacs packages (requires Internet connection)")
      "\n"))
   (fancy-splash-insert
    :face 'variable-pitch "To quit a partially entered command, type "