Add Scala Days conference data

[TerryFYL]

Summary

• Add Scala Days to the conference dataset.
• Use the public Scala Days site and the Scala Center announcement for the organizer/source reference.

/claim #4

Sources

• https://scaladays.org/
• https://www.scala-lang.org/blog/2026/03/26/scala-days-2026.html
• https://www.scala-lang.org/blog/2026/04/30/scala-days-2026-cfp.html

Validation

• Confirmed current main does not already contain Scala Days.
• Confirmed all source links return HTTP 200.
• Checked the added README table row has the expected column count.
• Ran git diff --check.

Demo Note

This is a documentation/data-only change to README.md; no runtime demo is applicable.

AI assistance disclosure

I used AI assistance to locate public sources, draft the dataset row, and run local validation. I reviewed the final change before submitting.