monitoring: better advice for alerts that do not have an entry in alert_solutions

Created by: bobheadxi

Alerts notifications currently sets up a link back to alert_solutions with an anchor, e.g.

https://docs.sourcegraph.com/admin/observability/alert_solutions#${service}-${alert}

This makes for an awkward situation when the alert has no associated entry (presumably, Solutions: "none"), where the link will just go to the top of the page - while there is a note at the top, it took me a while to see it.

I think we might want to update the generator to generate a minimal entry that says, "there's not direct solution for this alert, please open an issue via the bug report page" or similar 🤔

cc @slimsag