[DOCS] Provide an explanation of what dangling indices are and how they can occur #26008
Description
The elasticsearch docs make reference to dangling indices:
but there's no explanation of what they are or how they come about.
It would be good to have a glossary of some type to refer to for various terminology like this that could be expanded over time.
It would be good to link/reference Index Tombstones when documenting dangling indices too.