Update announcement banner for Postgres Marathon

Update announcement banner for Postgres Marathon

@bogdan

Changes

• Replace outdated Launch Week banner with current Postgres Marathon promotion
• Update banner text to "#PostgresMarathon is live: deep-dive technical insights every day"
• Link banner to /blog/tags/postgres-marathon tag page showing all marathon posts
• Change banner ID to postgres_marathon_banner for proper tracking

Benefits

• Promotes ongoing Postgres Marathon content series
• Drives traffic to all 6 marathon blog posts via tag aggregation
• Uses established X hashtag #PostgresMarathon for brand consistency
• Professional, engaging messaging without emojis

Files changed

• docusaurus.config.js - Updated announcementBar configuration