docs: Add `snap run --strace` discussion to the “Debug a Snap” Guide

[hillpd]

Adding a short section would help developers understand how to capture syscalls inside snap confinement. This functionality isn’t documented anywhere outside the snap(8) manpage, so surfacing it in the debugging guide would make it more discoverable.

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Reported from: https://documentation.ubuntu.com/snapcraft/stable/how-to/debugging/debug-a-snap/

[bepri]

Thanks for the report, @hillpd! It looks like this might've existed before, but it was lost in a recent overhaul/migration of our docs. There's a link at the bottom of the page ("Debugging snaps") that redirects right back to the page you created this ticket from. I'll bet that snap run --strace was referenced there, once upon a time.