doc: Include extend example in Secret Detection custom ruleset docs
What does this MR do and why?
Adds documentation example for Category:Secret Detection demonstrating how to extend the default configuration without overriding entirely.
This specific example additionally shows how to use gitleaks' allowList
to ignore certain secrets to be used in testcases. This inadvertently solves #370546 (comment 1372944230) but we may want that issue open for a more GL-native feature
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