man-pages: force encoding when reading doc sources

Force the encoding to not be dependent on the environment locale.

In particular on Windows, the encoding could default to an ANSI page
code, which would fail to load any file containing bytes 0x80+, i.e any
multi-byte UTF-8 character.

Closes #12748

doc_src/conf.py

@@ -218,7 +218,7 @@ latex_engine = "xelatex"
 
 def get_command_description(path, name):
     """Return the description for a command, by parsing its synopsis line"""
-    with open(path) as opened:
+    with open(path, encoding="utf8") as opened:
         for line in opened:
             if line.startswith(name + " - "):
                 _, desc = line.split(" - ", 1)