docs: document config ignore patterns

[cyberchen1995]

Summary

• document config.optional.ignore in Chinese and English READMEs
• add configuration docs for .gitignore-compatible ignore rules
• fix docs index links to the existing configuration pages

Test Plan

• env GOCACHE=/private/tmp/opensca-go-build-cache GOTMPDIR=/private/tmp go test . ./cmd/config ./opensca ./opensca/walk ./opensca/sca/filter
• git diff --check origin/master...HEAD