Doc: Fix links in the Extending Qt Creator Manual

- Hide links to sections that have not been written yet
- Fix some links to information that was moved
- Use HTTPS in links to Qt Creator Manual topics

Fixes: QTCREATORBUG-23529
Change-Id: Idd162c04fe6992bbecd2acd4a7ec388e49f29b84
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

doc/qtcreatordev/src/creating-plugins.qdoc

@@ -63,6 +63,7 @@
 
     \list
         \li \l{The Plugin Manager, the Object Pool, and Registered Objects}
+    \omit
         \li \l{Aggregations}
         \li \l{Extending and Providing Interfaces}
     \endlist
@@ -72,5 +73,6 @@
     \list
         \li \l{A Note on Binary Compatibility}
         \li \l{Creating User-Installable Plugins}
+    \endomit
     \endlist
 */