Add link to annual media developer survey.

This will be removed after the survey has closed in ~1 month.

PiperOrigin-RevId: 399890121

docs/index.md

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+The Android media team is interested in your experiences with the Android media
+APIs and developer resources. Please provide your feedback by
+[completing this short survey](https://goo.gle/media-survey-6).
+{:.info}
 
 ExoPlayer is an application level media player for Android. It provides an
 alternative to Android’s MediaPlayer API for playing audio and video both