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 which is blocked on [1].

Closes #573
Closes flatpak/flatpak#6330

[1]: flatpak/flatpak#4591

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>