Convert org.pkl.core.Release into a record (#812)

Also, add doc comments to some methods.

pkl-doc/src/main/kotlin/org/pkl/doc/CliDocGenerator.kt

@@ -49,7 +49,7 @@ class CliDocGenerator(private val options: CliDocGeneratorOptions) : CliCommand(
       sourceCodeUrlScheme =
         if (options.isTestMode)
           "https://github.com/apple/pkl/blob/0.24.0/stdlib%{path}#L%{line}-L%{endLine}"
-        else Release.current().sourceCode().sourceCodeUrlScheme,
+        else Release.current().sourceCode().sourceCodeUrlScheme(),
       documentation =
         if (options.isTestMode) URI("https://pages.github.com/apple/pkl/stdlib/pkl/0.24.0/")
         else

pkl-doc/src/main/kotlin/org/pkl/doc/DocScope.kt

@@ -210,7 +210,7 @@ internal class SiteScope(
         val path = "/stdlib/${name.substring(4)}.pkl"
         Release.current()
           .sourceCode()
-          .sourceCodeUrlScheme
+          .sourceCodeUrlScheme()
           .replaceSourceCodePlaceholders(path, sourceLocation)
           .toUri()
       }