Windows: reorganize localized documentation, each in a dedicated folder. Adapt installers and documentation URL logic.

Now we have 3 folders under html directory：en, ru, zh-cn. Similar separation exist in online version:
https://veracrypt.fr/en
https://veracrypt.fr/ru
https://veracrypt.fr/zh-cn

src/Setup/Setup.c

@@ -1156,6 +1156,14 @@ err:
 			FindClose (h);
 		}
 
+		// remove legacy folder "docs\en\ru" if present in installation directory
+		{
+			wchar_t folder[TC_MAX_PATH];
+			// since we've done SetCurrentDirectory(szDestDir), a relative path will be resolved correctly
+			StringCbCopyW(folder, sizeof(folder), L"docs\\html\\en\\ru");
+			StatRemoveDirectory(folder);
+		}
+
 		// remove language XML files from previous version if any
 		h = FindFirstFile (L"Language*.xml", &f);