install/windows: create directory for existing google links

- Move the Windows install files to a separate 'windows' subdirectory.

Google query for 'vcpkg curl' returns a bad link (404)
https://www.google.com/search?q=vcpkg+curl
https://everything.curl.dev/install/windows/win-vcpkg.html

mdBook had generated it without 'windows/' since that wasn't a
subdirectory.
https://everything.curl.dev/install/win-vcpkg.html

Closes #xxxxx

SUMMARY.md

@@ -26,9 +26,9 @@
   * [HTTP basics](protocols/http.md)
 * [Install curl and libcurl](install/README.md)
   * [Linux](install/linux.md)
-  * [Windows](install/windows.md)
-    * [MSYS2](install/win-msys2.md)
-    * [vcpkg](install/win-vcpkg.md)
+  * [Windows](install/windows/README.md)
+    * [MSYS2](install/windows/win-msys2.md)
+    * [vcpkg](install/windows/win-vcpkg.md)
   * [macOS](install/macos.md)
   * [Container](install/container.md)
 * [Source code](source/README.md)

install/README.md

@@ -13,6 +13,6 @@ there.
 Commonly, libcurl is installed at the same time as curl.
 
   * [Linux](linux.md)
-  * [Windows](windows.md)
+  * [Windows](windows/)
   * [macOS](macos.md)
   * [Container](container.md)