Updated overview.md

- Link John's ForestDB requirement doc to Storage Managment
- Link latest recovery document from John
- Added description for Metadata Managment and linked to Index Build Prepare phase
  to show an example

Change-Id: I6c01ef9b83c3c2177438489bacdf862a33394be6
Reviewed-on: http://review.couchbase.org/34841
Reviewed-by: John Liang <john.liang@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>