doc: Support any link having a fragment except http(s) links for linking between headings

modules/doc/doc/1-module-documentation.md

@@ -36,10 +36,18 @@ For linking between headings, place an anchor where you want to link to, e.g.:
 
     # <a id="heading"></a> Heading
 
+Please note that anchors have to be unique across all your Markdown documentation files.
+
 Now you can reference the anchor either in the same or **in another** Markdown documentation file, e.g.:
 
     This is a link to [Heading](#heading).
 
+Other tools support linking between headings by giving the filename plus the anchor to link to, e.g.:
+
+    This is a link to [About/Heading](1-about.md#heading.md)
+
+This syntax is also supported in Icinga Web 2.
+
 ## <a id="images"></a> Including Images
 
 Images must placed in the `img` directory beneath your module's `public` directory, e.g.:

modules/doc/library/Doc/SectionRenderer.php

@@ -232,7 +232,7 @@ class SectionRenderer extends Renderer
                 $html
             );
             $content[] = preg_replace_callback(
-                '/<a\s+(?P<attribs>[^>]*?\s+)?href="#(?P<fragment>[^"]+)"/',
+                '/<a\s+(?P<attribs>[^>]*?\s+)?href="(?:(?!http:\/\/)[^#]*)#(?P<fragment>[^"]+)"/',
                 array($callback, 'render'),
                 $html
             );