Change default version to "Latest"

If a version was not set in the project table, and the project isn't a
Python package, then default to a string "Latest" for the Sphinx
version. "None" isn't a valid value for Sphinx.

src/documenteer/conf/_toml.py

@@ -381,15 +381,15 @@ def version(self) -> Optional[str]:
 
         1. project.version field in ``documenteer.toml``
         2. From importlib if the project.python table is set
-        3. Default is None.
+        3. Default is "Latest".
         """
         if self.conf.project.version is not None:
             return self.conf.project.version
         elif self.conf.project.python is not None:
             # Via pydantic validation we know this works
             return get_version(self.conf.project.python.package)
         else:
-            return None
+            return "Latest"
 
     @property
     def rst_epilog_path(self) -> Optional[Path]: