docs(pipelines): add examples for pipeline variables (aws#19288)

Fixes aws#19184.


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/pipelines/lib/codepipeline/codebuild-step.ts

@@ -234,6 +234,20 @@ export class CodeBuildStep extends ShellStep {
    * it finishes its `post_build` phase.
    *
    * @param variableName the name of the variable for reference.
+   * @example
+   * // Access the output of one CodeBuildStep in another CodeBuildStep
+   * declare const pipeline: pipelines.CodePipeline;
+   *
+   * const step1 = new pipelines.CodeBuildStep('Step1', {
+   *   commands: ['export MY_VAR=hello'],
+   * });
+   *
+   * const step2 = new pipelines.CodeBuildStep('Step2', {
+   *   env: {
+   *     IMPORTED_VAR: step1.exportedVariable('MY_VAR'),
+   *   },
+   *   commands: ['echo $IMPORTED_VAR'],
+   * });
    */
   public exportedVariable(variableName: string): string {
     if (this.exportedVarsRendered && !this.exportedVariables.has(variableName)) {

packages/@aws-cdk/pipelines/lib/codepipeline/codepipeline-source.ts

@@ -162,6 +162,19 @@ export abstract class CodePipelineSource extends Step implements ICodePipelineAc
    *   - `RegistryId`
    *
    * @see https://docs.aws.amazon.com/codepipeline/latest/userguide/reference-variables.html#reference-variables-list
+   * @example
+   * // Access the CommitId of a GitHub source in the synth
+   * const source = pipelines.CodePipelineSource.gitHub('owner/repo', 'main');
+   *
+   * const pipeline = new pipelines.CodePipeline(scope, 'MyPipeline', {
+   *   synth: new pipelines.ShellStep('Synth', {
+   *     input: source,
+   *     commands: [],
+   *     env: {
+   *       'COMMIT_ID': source.sourceAttribute('CommitId'),
+   *     }
+   *   })
+   * });
    */
   public sourceAttribute(name: string): string {
     return makeCodePipelineOutput(this, name);