add link to alternative scan options

[s-santillan]

Thanks for improving Semgrep Docs 😀

Please ensure

• [ ] A subject matter expert (SME) reviews the content
• [ ] A technical writer reviews the content or PR
• [ ] This change has no security implications or else you have pinged the security team
• [ ] Redirects are added if the PR changes page URLs
• [ ] If you have changed any header tag links (doc/#this-kind-of-anchor), update all instances of that link

This PR is up for discussion!!

See the Slack thread.

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[s-santillan]

It turns out that I never set you as reviewer, @khorne3 - sorry about that.