fix #6583, fix #14376, index+search now generated for all projects, many bug fixes with nim doc (#14324)

* refs #6583 fix nim doc output
* changelog
* change default for outDir when unspecified
* cleanups
* --project implies --index

nimdoc/testproject/expected/subdir/subdir_b/utils.html

@@ -20,7 +20,7 @@
 <title>subdir/subdir_b/utils</title>
 <link rel="stylesheet" type="text/css" href="../../nimdoc.out.css">
 
-<script type="text/javascript" src="dochack.js"></script>
+<script type="text/javascript" src="../../dochack.js"></script>
 
 <script type="text/javascript">
 function main() {
@@ -83,6 +83,9 @@ function main() {
   </div>
   <div id="global-links">
     <ul class="simple">
+    <li>
+      <a href="../../theindex.html">Index</a>
+    </li>
     </ul>
   </div>
   <div id="searchInputDiv">

nimdoc/testproject/expected/testproject.html

@@ -83,6 +83,9 @@ function main() {
   </div>
   <div id="global-links">
     <ul class="simple">
+    <li>
+      <a href="theindex.html">Index</a>
+    </li>
     </ul>
   </div>
   <div id="searchInputDiv">