website: update backtrace explanations

[Simon Frei]

I added information about how to get a backtrace when using an appimage.
To do so I reordered the explanations a bit. Please have a look whether
it is comprehensible. Also a more extensive explanation of what you
actually need to do on windows would be useful, but I have no idea about
that.
diff:
https://commits.kde.org/websites/digikam-org/c5226a3868e5b36a6ddb3c677da5f16594027a52
staging: https://staging.digikam.org/contribute/