Update the documentation about the `/{db}/_explain` endpoint

[pgj]

Please note that the /{db}/_explain endpoint gained some extensions a couple of months ago to aid users in understanding the index selection logic better. This has been documented by Apache CouchDB and it should be added to the IBM Cloud documentation as well:

https://docs.couchdb.org/en/latest/api/database/find.html#index-selection

Glynn wrote a nice blog entry about the whole feature:

https://blog.cloudant.com/2024/01/17/Explaining-Explain.html

[pgj]

@ricellis noted that the API documentation was updated: https://cloud.ibm.com/apidocs/cloudant#postexplain-response — due to that, this ticket should be more about adding a section on this topic to the collection of "How to" documents. Otherwise it might go unnoticed for the customers.