docs: teach users how to get onto the Nightly build

[0xSage]

Pages

• Page(s) that need to be done
• https://jan.ai/install/nightly (right before install-from-source)

Success Criteria Content that should be covered

• Warn that nightly is highly unstable but can be used try out new features
• Available here https://github.com/janhq/jan?tab=readme-ov-file#download and via Discord channel
• Once you download the nightly, it will autoprompt you to update each time the app is restarted
• Some users might want 2 versions (stable vs nightly), teach them how
• Other helpful tidbits exist

Additional context Examples, reference pages, resources

• Keep it concise, factual, use screenshots/visuals when needed