Import Mono.Android index.xml

Reintroduces [index.xml][0] to allow docs to continue to be exported
via the `mdoc export-msxdoc` command.

[0]: https://github.com/xamarin/android-api-docs/blob/main/a993a8996fe6dc94bea8807893f0cf933847a30e/Mono.Android/en/index.xml

Author: pjcollins