Add IntelliJ troubleshooting tips to BUILD.md by marioguzzzman · Pull Request #1096 · processing/processing4

This PR adds a new section to BUILD.md with troubleshooting tips for using IntelliJ IDEA, including:

• Selecting the correct JDK (temurin-17)
• Steps to download the right JDK via IntelliJ
• Explaining the “Duplicate content roots detected” warning

Screenshots included for clarity.

Thanks again @Stefterv and @SableRaf for the green light!

Closes #1095