Improve documentation on TRE tear down

[jonnyry]

The existing documentation on TRE tear down doesn't make clear the difference between using make tre-destroy and az group delete:

https://microsoft.github.io/AzureTRE/v0.15.2/tre-admins/tear-down/

az group delete despite deleting the resource groups, leaves behind a number of resources such as the key vault, diagnostic log settings and others, which causes a subsequent deployment of the TRE to fail multiple times when deploying terraform.

make tre-destroy remedies this by seperately deleting the resources left behind as can be seen in destroy_env_no_terraform.sh.

The documentation would be more helpful to first time users, if the pitfalls of manually deleting the resources groups were pointed out. I notice a number of tickets where people have fallen into this treap and then have deployed to a new TRE_ID to work around the issue.