Include schema.org structured data

[tong]

Add structured data as json-ld.

Data types:

• [x] index: https://schema.org/Organization
• [x] program/event: https://schema.org/Event or https://schema.org/MusicEvent
• [ ] program/event(virtual): https://schema.org/BroadcastEvent
• [x] artist: https://schema.org/artist
• [x] recording: https://schema.org/MusicRecording
• [x] records/releases: https://schema.org/MusicRelease

Check https://schema.org/

[tong]

Improve event items as proposed by google search console.

Top non-critical issues‡

• Missing field "offers"
• Missing field "image"
• Missing field "description"