[Azure docs] Document hosting plans for Azure Functions metrics 

On the [Azure Functions](https://docs.elastic.co/integrations/azure_functions) doc page:


1. Create the new section **Hosting plans** and put it at the same level of **Data streams**
2. Reuse content drafted in #6086 ([first two bullets](https://github.com/elastic/integrations/issues/6704#issuecomment-1675434444))

CC @zmoog 