chore: add webinar banner

[gminn]

Summary

We have another webinar coming up -- re-add the banner with the sign up link.

Test Plan

Visual check + link check

[cloudflare-pages[bot]]

Deploying interrupt with    Cloudflare Pages

Latest commit:	11e01b8
Status:	✅  Deploy successful!
Preview URL:	https://ce6e3755.interrupt.pages.dev
Branch Preview URL:	https://gminn-update-banner-for-webi.interrupt.pages.dev

View logs

[gminn]

• #509 👈
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.

Join @gminn and the rest of your teammates on Graphite

[gminn]

Merge activity

• Aug 21, 9:57 AM EDT: @gminn started a stack merge that includes this pull request via Graphite.
• Aug 21, 9:57 AM EDT: @gminn merged this pull request with Graphite.