Document how to get useful backtraces

[tintou]

Bug reports would be much better with proper crash logs, it would also probably help the 3rd party developers to have an easy way to get backtraces.

Here is an example of the steps:
elementary/wingpanel#416 (comment)

[danirabbit]

We have bt and logs guides on the contributor docs: https://docs.elementary.io/contributor-guide/feedback/reporting-issues/inspecting-crashes and https://docs.elementary.io/contributor-guide/feedback/reporting-issues/debug-logs

It would probably be better to update or revise those pages

[marbetschar]

I documented the debugging steps as well on my website. Feel free to copy & paste as you see fit:
https://www.marco.betschart.name/areas/elementary-os/debug-vala-or-c-app