Fix docs build on Windows

Sphinx expects the "document name" field to be normalized with respect
to slashes (all /), so we now do this in a portable way for Windows.

docs/html/conf.py

@@ -273,17 +273,26 @@
     )
 ]
 
+
+def to_document_name(path, base_dir):
+    """Convert a provided path to a Sphinx "document name".
+    """
+    relative_path = os.path.relpath(path, base_dir)
+    root, _ = os.path.splitext(relative_path)
+    return root.replace(os.sep, '/')
+
+
 # Here, we crawl the entire man/commands/ directory and list every file with
 # appropriate name and details
-man_dir = os.path.join(docs_dir, 'man/')
+man_dir = os.path.join(docs_dir, 'man')
 raw_subcommands = glob.glob(os.path.join(man_dir, 'commands/*.rst'))
 if not raw_subcommands:
     raise FileNotFoundError(
         'The individual subcommand manpages could not be found!'
     )
 for fname in raw_subcommands:
-    fname_base = fname[len(man_dir):-4]
-    outname = 'pip-' + fname_base[9:]
+    fname_base = to_document_name(fname, man_dir)
+    outname = 'pip-' + fname_base.split('/')[1]
     description = u'description of {} command'.format(
         outname.replace('-', ' ')
     )