Reorganized tests and added tests for _doc.

[dblock]

Description

1. Reorganized tests to match spec.
2. Added tests for POST/PUT/DELETE _doc, adding the missing 201 response.

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[github-actions]

Changes Analysis

Commit SHA: cf8d641
Comparing To SHA: 362c90c

API Changes

Summary

├─┬Paths
│ ├─┬/{index}/_doc/{id}
│ │ ├─┬PUT
│ │ │ └─┬Responses
│ │ │   └──[➕] codes (23592:7)
│ │ └─┬POST
│ │   └─┬Responses
│ │     └──[➕] codes (23592:7)
│ └─┬/{index}/_doc
│   └─┬POST
│     └─┬Responses
│       └──[➕] codes (23592:7)
└─┬Components
  └──[➕] responses (23592:7)

Document Element	Total Changes	Breaking Changes
paths	3	0
components	1	0

• Total Changes: 4
• Additions: 4

Report

The full API changes report is available at: https://github.com/opensearch-project/opensearch-api-specification/actions/runs/9465079248/artifacts/1589424287

API Coverage

	Before	After	Δ
Covered (%)	476 (46.62 %)	476 (46.62 %)	0 (0 %)
Uncovered (%)	545 (53.38 %)	545 (53.38 %)	0 (0 %)
Unknown	24	24	0

[nhtruong]

the operations tested in the index lifecycle story is actually part of the indices namespace. I think we should put it in tests/indices/lifecycle.yaml

[dblock]

Makes sense to organize tests in namespaces, updated and added to DEVELOPER_GUIDE.