Add Composite aggregation page. (#9479)

* Add Composite aggregation page.

Signed-off-by: Dave Welsch <dwelsch@expertsupport.com>

* Doc review

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Apply suggestions from code review

Signed-off-by: Nathan Bower <nbower@amazon.com>

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Signed-off-by: Dave Welsch <dwelsch@expertsupport.com>
Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>
Signed-off-by: Nathan Bower <nbower@amazon.com>
Co-authored-by: Fanit Kolchina <kolchfa@amazon.com>
Co-authored-by: Nathan Bower <nbower@amazon.com>

Author: 3 people