Include schema.org structured data

[tong]

Add structured data as json-ld.

Data types:

• index: https://schema.org/Organization
• program/event: https://schema.org/Event or https://schema.org/MusicEvent
• program/event(virtual): https://schema.org/BroadcastEvent
• artist: https://schema.org/artist
• recording: https://schema.org/MusicRecording
• 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"