Add key to frontmatter

[danshafie]

Description

Adding frontmatter key so we can know on the developer site if and what diagram to render

New Pattern Checklist

• [ ] Pattern file name should follow docs/pattern-category/pattern-name.md
• [ ] Create an on-brand diagram (https://docs.google.com/presentation/d/1Zf256Z6fBvre3uclIbmxXsDpnTIxiBX66b13pHbGIYc/edit?usp=sharing)
• [ ] Implement ksqlDB or code example (not all patterns will have code, but highly desired)
• [ ] Provide references
• [ ] Validate hyperlinks
• [ ] Spell check (e.g. using aspell)