Update Data Prepper opensearch sink documentation (#6386)

* Update Data Prepper opensearch sink documentation

Signed-off-by: Taylor Gray <[email protected]>

* Formatting fixes and adding introductory text.

Signed-off-by: Naarcha-AWS <[email protected]>

* Apply suggestions from code review

Signed-off-by: Naarcha-AWS <[email protected]>

* Update opensearch.md

Signed-off-by: Naarcha-AWS <[email protected]>

* Apply suggestions from code review

Signed-off-by: Naarcha-AWS <[email protected]>

* Update opensearch.md

Signed-off-by: Naarcha-AWS <[email protected]>

* Update opensearch.md

Signed-off-by: Naarcha-AWS <[email protected]>

* Update opensearch.md

Signed-off-by: Naarcha-AWS <[email protected]>

* Update opensearch.md

Signed-off-by: Naarcha-AWS <[email protected]>

* Apply suggestions from code review

Co-authored-by: Nathan Bower <[email protected]>
Signed-off-by: Naarcha-AWS <[email protected]>

* Apply suggestions from code review

Co-authored-by: Nathan Bower <[email protected]>
Signed-off-by: Naarcha-AWS <[email protected]>

* Apply suggestions from code review

Signed-off-by: Naarcha-AWS <[email protected]>

---------

Signed-off-by: Taylor Gray <[email protected]>
Signed-off-by: Naarcha-AWS <[email protected]>
Co-authored-by: Naarcha-AWS <[email protected]>
Co-authored-by: Nathan Bower <[email protected]>