Adjust documentation for Sphinx 4.0.

Discussed upstream at <https://github.com/ganeti/ganeti/pull/1615>.

diff --git a/doc/index.rst b/doc/index.rst
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -119,7 +119,6 @@ Draft designs
    design-hotplug.rst
    design-internal-shutdown.rst
    design-kvmd.rst
-   design-location.rst
    design-linuxha.rst
    design-location.rst
    design-lu-generated-jobs.rst
diff --git a/lib/build/shell_example_lexer.py b/lib/build/shell_example_lexer.py
--- a/lib/build/shell_example_lexer.py
+++ b/lib/build/shell_example_lexer.py
@@ -79,4 +79,4 @@ class ShellExampleLexer(RegexLexer):
 
 
 def setup(app):
-  app.add_lexer("shell-example", ShellExampleLexer())
+  app.add_lexer("shell-example", ShellExampleLexer)