Add maintainers to MAINTAINERS.md, extract RESPONSIBILITIES.

[dblock]

Signed-off-by: Daniel (dB.) Doubrovkine dblock@amazon.com

Description

• Clarifies admins vs. maintainers by updating the list of admins.
• Extracts RESPONSIBILITIES from MAINTAINERS.
• Updates the list of MAINTAINERS and CODEOWNERS to match actual permissions.

Issues Resolved

Closes https://github.com/opensearch-project/.github/issues/99.

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