Docs: Sharing Build Logic using buildSrc

A Gradle-reserved directory for build-logic Kotlin code that needs to live outside the .gradle.kts build scripts.

• Extracting reusable build logic.
  • NOTE: The overhead for one-off scripts is not worthwhile
• Extracting build logic to support the Gradle Configuration Cache
  • Closures inside .kts files can capture an implicit this reference, which is CC-incompatible.
• Defining a convention plugin to reduce duplication in .kts files