In kibana, how to delete all the documents in an Index without deleting the index from Dev tools

You could use

POST /index-name-000001/_delete_by_query
{
  "query": {
    "match_all": {}
  }
}

Further information is provided in https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-delete-by-query.html

If it's a very large index add,?conflicts=proceed, so it's run on the background.

POST my-index-000001/_delete_by_query?conflicts=proceed
{
  "query": {
    "match_all": {}
  }
}