portal-api-reference: Redirect to x-d-p docs

Flatpak portal docs are duplicated in libflatpak api reference so
nothing is lost but we really need to figure out how to update the
libflatpak API docs

Closes #573
Closes flatpak/flatpak#6330

Author: bbhtt

docs/portal-api-reference.rst

@@ -4,4 +4,6 @@ Portal API Reference
 ========================
 
 .. raw:: html
-   :file: portal-docs.html
+
+   <meta http-equiv="refresh" content="0; url=https://flatpak.github.io/xdg-desktop-portal/docs/" />
+   <script>window.location.href = 'https://flatpak.github.io/xdg-desktop-portal/docs/'</script>