Fortigate: Update docs to replace alert gim category with detection

[CG3827]

Notes for Reviewers

• The commit history must be preserved - please use the rebase-merge or standard merge option instead of squash-merge
• Sync up with the author before merging

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
6 out of 7 committers have signed the CLA.

✅ jordan-linden
✅ annie-zempel
✅ StefanAustin
✅ miwent
✅ CG3827
✅ gormanbj
❌ bkwins-gl
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