feat: add Discord community links and automation
- Add Discord invite links to README, docs navigation, docs home, About page, and version card
- Embed the Discord server widget on the docs home page and in the JellyGlance About page
- Add Discord release notifications using DISCORD_RELEASES_WEBHOOK
- Add hourly GitHub star growth notifications using DISCORD_STARS_WEBHOOK
- Add shared Discord embed notification script for release and star posts
- Document Discord webhook setup and required GitHub Actions secrets
- Validate workflows, notifier script, docs build, and web build