Addon Manager: Fix relative external links

src/Mod/AddonManager/addonmanager_readme_controller.py

@@ -63,6 +63,7 @@ class ReadmeController(QtCore.QObject):
         self.stop = True
         self.widget = widget
         self.widget.load_resource.connect(self.loadResource)
+        self.widget.follow_link.connect(self.follow_link)
 
     def set_addon(self, repo: Addon):
         """Set which Addon's information is displayed"""
@@ -152,6 +153,16 @@ class ReadmeController(QtCore.QObject):
             NetworkManager.AM_NETWORK_MANAGER.abort(request)
         self.resource_requests.clear()
 
+    def follow_link(self, url: str) -> None:
+        final_url = url
+        if not url.startswith("http"):
+            if url.endswith(".md"):
+                final_url = self._create_markdown_url(url)
+            else:
+                final_url = self._create_full_url(url)
+        FreeCAD.Console.PrintLog(f"Loading {final_url} in the system browser")
+        QtGui.QDesktopServices.openUrl(final_url)
+
     def _create_full_url(self, url: str) -> str:
         if url.startswith("http"):
             return url
@@ -160,6 +171,11 @@ class ReadmeController(QtCore.QObject):
         lhs, slash, _ = self.url.rpartition("/")
         return lhs + slash + url
 
+    def _create_markdown_url(self, file: str) -> str:
+        base_url = utils.get_readme_html_url(self.addon)
+        lhs, slash, _ = base_url.rpartition("/")
+        return lhs + slash + file
+
 
 class WikiCleaner(HTMLParser):
     """This HTML parser cleans up FreeCAD Macro Wiki Page for display in a