draft: update rule syntax docs

[brandonspark]

Draft PR for updating rule syntax docs, in anticipation of the long-away date where we officially migrate to the new syntax.

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

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