fix(chart/data-prepper) Remove demo pipeline from values, add explicit demo value

[janhoy]

Description

This PR comments demo pipline from values, and also removes the alread-commented complex pipeline example, refering to documentation instead.

Now, when installing the chart with defaults only, the install will fail with this message:

helm.go:86: 2024-12-17 15:21:55.866789 +0100 CET m=+0.033838001 [debug] execution error at (data-prepper/templates/deployment.yaml:98:27): A valid pipeline configuration is required! Please set one of:
pipelineConfig.existingSecret - for your custom pipeline secret
pipelineConfig.enabled - for inline configuration in values.yaml or
pipelineConfig.demoPipeline=true - for a sample demo pipeline using random source and stdout sink.

If you instead install the chart with --set pipelineConfig.demoPipeline=true, the behavior will be identical to current chart behavior, and we also print an informative message in stdout about what pipeline configuration is in use:

...
The demo pipeline configuration is enabled, using the `random` source and `stdout` sink.

Issues Resolved

Fixes #624

Check List

• Commits are signed per the DCO using --signoff

For any changes to files within Helm chart directories:

• Helm chart version bumped
• Helm chart CHANGELOG.md updated to reflect change

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