add links from the introduction into the documentation

doc/index.html

@@ -118,16 +118,21 @@
 
     <ul>
     <li>
-        is designed to run on any Unix system
+        is designed to
+        <a href="#prerequisites">run on any Unix system</a>
     </li>
     <li>
         is easy to adapt and to extend
     </li>
     <li>
-        builds many free libraries in addition to the cross compiler
+        builds
+        <a href="#packages">many free libraries</a>
+        in addition to the cross compiler
     </li>
     <li>
-        can also build just a subset of the packages,
+        can also
+        <a href="#usage">build just a subset</a>
+        of the packages,
         and automatically builds their dependencies
     </li>
     <li>
@@ -142,7 +147,8 @@
         thus ensuring the whole build mechanism is transparent
     </li>
     <li>
-        allows inter-package and intra-package parallel builds
+        allows inter-package and intra-package
+        <a href="#usage">parallel builds</a>
         whenever possible
     </li>
     <li>