[feature] Add Doris Summit 26 promotion

[morningman]

Summary

• add a Doris Summit 26 navbar action that opens the official event site in a new tab
• add responsive mobile presentation, subtle shimmer motion, and reduced-motion support
• add a Doris Summit 26 section below Join Slack in the community nudge, while keeping the docs issue section last on documentation pages
• update mobile sticky offsets and the Ask Me modal clearance for the taller navbar

Validation

• node --test src/components/home-next/SlackCommunityNudge.logic.test.js
• git diff --check
• full build not run