[DOC] Create documentation for _index APIs

[kolchfa-aws]

What do you want to do?

• Request a change to existing documentation
• [x ] Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.

 PUT /<index>/_create/
POST /<index>/_create/
HEAD /<index>/_doc/<id>
HEAD /<index>/_source/<id>
GET /<index>/_source/<id> // deprecated (per Nate)
PUT /<index>/_block/<block>
HEAD /<index>

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

[Naarcha-AWS]

• PUT /<index>/_create/, POST /<index>/_create/,HEAD /<index>/_doc/<id> are documented here :https://opensearch.org/docs/latest/api-reference/document-apis/index-document/
• HEAD /<index>/_source/<id>, GET /<index>/_source/<id> is documented but missing important description information: https://opensearch.org/docs/latest/api-reference/document-apis/get-documents/
• PUT /<index>/_block/<block> is missing
• HEAD /<index>: Exists https://opensearch.org/docs/latest/api-reference/index-apis/exists/