doc: fix a bunch of broken links.

doc/credits.dox

@@ -187,8 +187,8 @@ Are the below lists missing your name or something's wrong?
     @ref Platform::GlfwApplication crash fixes
 -   **[\@Selot](https://github.com/Selot)** --- Fixes in the `base-qt`
     bootstrap project
--   **Séverin Lemaignan** ([\@severin-lemaignan](severin-lemaignan/)) --- lots
-    of feedback and bug reports for the initial releases
+-   **Séverin Lemaignan** ([\@severin-lemaignan](https://github.com/severin-lemaignan))
+    --- lots of feedback and bug reports for the initial releases
 -   **[\@sigman78](https://github.com/sigman78)** --- Initial MSVC 2017 port,
     Vcpkg packages
 -   **Siim Kallas** ([\@seemk](https://github.com/seemk)) ---

doc/namespaces.dox

@@ -631,6 +631,9 @@ See @ref building, @ref cmake and @ref vulkan for more information.
 /** @dir MagnumPlugins/AnyImageImporter
  * @brief Plugin @ref Magnum::Trade::AnyImageImporter
  */
+/** @dir MagnumPlugins/AnySceneConverter
+ * @brief Plugin @ref Magnum::Trade::AnySceneConverter
+ */
 /** @dir MagnumPlugins/AnySceneImporter
  * @brief Plugin @ref Magnum::Trade::AnySceneImporter
  */