[DOCS] Add reindex API examples

[lcawl]

Relates to https://github.com/elastic/docs-projects/issues/302

This PR:

• Removes API reference sections from https://www.elastic.co/docs/reference/elasticsearch/rest-apis/reindex-data-stream
• Adds a migrated version of https://www.elastic.co/guide/en/elasticsearch/reference/8.18/docs-reindex.html (with the API reference details removed) since the amount of explanation required for each example seems more reasonable there than in https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-reindex

[lcawl]

Closing since there ought to be a bigger discussion about long-term home for this content.