Doc: Fix qdoc warnings

- Mark undocumented but existing functions with \c (instead of \l or \sa)
- Remove reference to functions I could no longer find
- Fix other broken links
- Fix reference to an image whose file format changed to WEBP
- Use {} instead of "" to mark alt text for images
- Add missing \a commands
- Add class name to a function name so that it can be found

Change-Id: I10655bb0356c7417ab0e14a3ce620930f4ee8349
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

doc/qtcreatordev/src/plugin-lifecycle.qdoc

@@ -28,8 +28,7 @@
             tracks the state of the plugin.
             You can get the \l{ExtensionSystem::PluginSpec} instances via the
             plugin manager's \l{ExtensionSystem::PluginManager::plugins()}{plugins()}
-            function, or, after a plugin is loaded, through the plugin's
-            \l{ExtensionSystem::IPlugin::pluginSpec()}{pluginSpec()} function.
+            function.
 
         \li Sets the plugins to \c Read state.