DOCS-1126 sharding intro
DOCS-1126 initial reorg of sharding content

Signed-off-by: Sam Kleinman <samk@10gen.com>
sh-reorg: second pass at editing and reorganization
DOCS-1126 sharding reorg, clarify chunk and chunk of documents

Signed-off-by: Sam Kleinman <samk@10gen.com>
sh-reorg: edits to high level content.
DOCS-1126 sharding-reorg add images