Update some link redirects

Author: bbhtt

docs/conventions.rst

@@ -366,7 +366,7 @@ convenience functions for accessing XDG base directories. These include:
   the ``g_get_user_cache_dir ()``, ``g_get_user_data_dir ()``,
   ``g_get_user_config_dir ()`` functions.
 - Qt: XDG base directories can be accessed with the `QStandardPaths
-  Class <https://doc.qt.io/qt-5/qstandardpaths.html>`_.
+  Class <https://doc.qt.io/archives/qt-5.15/qstandardpaths.html>`_.
 
 However, applications that aren't using one of these toolkits can expect to
 find their XDG base directories in the following locations:

docs/manifests.rst

@@ -261,7 +261,7 @@ Modules can be built with a variety of build systems, including:
 - `cmake <https://cmake.org/>`_
 - `cmake-ninja <https://cmake.org/cmake/help/v3.0/generator/Ninja.html>`_
 - `meson <https://mesonbuild.com/>`_
-- `qmake <https://doc.qt.io/qt-5/qmake-overview.html>`_
+- `qmake <https://doc.qt.io/archives/qt-5.15/qmake-overview.html>`_
 
 A "simple" build method is also available, which allows a series of
 commands to be specified.