docs: convert source_suffix to map

Since sphinx-doc 1.8.0 the source_suffix list is replaced by a map.
In sphinx 3.0 this is still automatically converted, but results in a
warning. Hence, switch to the new format.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/conf.py

@@ -53,7 +53,7 @@
 # You can specify multiple suffix as a list of string:
 #
 # source_suffix = ['.rst', '.md']
-source_suffix = ['.rst']
+source_suffix = {'.rst': 'restructuredtext'}
 
 # The encoding of source files.
 #