[DOCS]: Move _reindex API examples to ES reference documentation (elastic#129341)

* Add Reindex indices examples

* Update docs/reference/elasticsearch/rest-apis/reindex-indices.md

Co-authored-by: Liam Thompson <[email protected]>

* Update section structure and add TOC

* Add back the TEST snippets

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Co-authored-by: Liam Thompson <[email protected]>

Author: charlotte-hoblik