AzureMonitor: docs for multiple subscriptions (#17194)

* AzureMonitor: docs for multiple subscriptions

Also, links to images in Grafana.org instead of the images
in the plugin GitHub repo.

* Update docs/sources/features/datasources/azuremonitor.md

Co-Authored-By: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Update docs/sources/features/datasources/azuremonitor.md

Co-Authored-By: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* docs: fix auto format errors

VS code automatically escapes the dollar sign.