[Feature Request] add more alerting "default" templates

So, this is what caused me to give up for 1 year to move to grafana unified alerting:

Here is a default slack notification from grafana 8 with a basic alert:
This shows in very clear ways:
- where this happened
- which alert
- the value that broke the threshold
All done by defining slack as destination and creating a simple alert.

While the new alerting:
If I do the same, I get an ugly alert

After working a lot, I managed to implement a generic template ... but I need to define each alert to emit the exact right data.
This is quite brittle and quite hard to maintain when having thousands of alerts.

=> would it be possible to see if a default template could be added that would look like the old one ? ( I guess it might be difficult - so maybe just for "classic_conditions" alerts?)
We could then define a receiver for those alerts, making life so much easier for people starting with grafana alerting or migrating

Start by locating the default notification-template handling and the code paths for classic_conditions alerts and receivers. Compare the current output with the linked Grafana 8 example and determine the required default fields. Done means a maintainable default template is defined for the requested alert type and its rendered notification is covered by appropriate tests.

Written by the indexing model from the issue text.