Add Scala Days to conference data tracker

[junbuilds96]

Summary

/claim #4

Adds Scala Days to the conference data tracker using public Scala Days / Scala language sources.

Source notes

• Conference page: https://scaladays.org/
• Organizer/source note: https://www.scala-lang.org/blog/2026/03/26/scala-days-2026.html
• I did not find public evidence that Scala Days signs the Speaker Engagement Agreement, so this uses No conservatively per the issue guidance.
• I did not find public travel reimbursement or honorarium commitments, so reimbursement is No and compensation is Never.

Validation

• Preserved the existing Markdown table schema.
• Inserted Scala Days before ScalarConf in the current table order.
• Ran git diff --check -- README.md.