feat(docs): add section about bumping Cluster imageName using renovate

[DerRockWolf]

This regex manager example can be used by users of renovate that want to also automate updates of their CNPG clusters.

Resolves #257

[acuntex]

I've been running this against CNPG tags and hit two practical issues with the current snippet:

1. Digest is mandatory in the regex
   The pattern requires @sha256:.... If a file has only a tag (no digest), Renovate can't extract -> won't open a PR. Making the digest optional lets Renovate both introduce a digest and refresh it later. The README's example currently requires the digest.

2. File pattern + robustness

• Consider matching both .yaml and .yml (and optional .j2), not only .yaml.
• It's safer if the tag capture stops before @ (digest) and before inline comments #.

Also, it helps to add a replaceStringTemplate that writes back the new tag and (optional) digest.

Proposed doc snippet (drop-in):

{
  customManagers: [
    {
      // CloudNativePG Cluster imageName
      customType: "regex",
      description: ["Process CloudnativePG Postgresql version"],
      managerFilePatterns: ["/\\.ya?ml(?:\\.j2)?$/"],
      matchStrings: [
        "imageName:\\s*(?<depName>[^:\\s#]+):(?<currentValue>[^@\\s#]+)(?:@(?<currentDigest>sha256:[A-Fa-f0-9]{64}))?"
      ],
      datasourceTemplate: "docker",
      versioningTemplate: "regex:^(?<major>\\d+)\\.(?<minor>\\d+)-(?<patch>\\d+)(?:-(?<compatibility>\\S+))?$",
      replaceStringTemplate: "imageName: {{depName}}:{{newValue}}{{#if newDigest}}@{{newDigest}}{{/if}}"
    }
  ]
}

Why this is safer:

• Digest is optional and strictly validated (sha256:[A-Fa-f0-9]{64}), so extraction works before/after pinning.
• currentValue stops at @ and #, so the digest and comments aren't swallowed.
• managerFilePatterns covers .yaml/.yml and templated .j2.
• replaceStringTemplate avoids Renovate writing only the tag and dropping the digest by accident.

[DerRockWolf]

@acuntex although your answer smells quite a bit AI (correct me if I'm wrong) I'm still going to address a few points.

Firstly, this PR just adds an example renovate manager, so if you need something more specific feel free to adjust it in your config.

The official file extension is .yaml. If you have any other extensions in use, like suggested by your template example, please adapt the config for your own setup. .j2 isn't the only templating "format" out there, so adding just one makes not much sense.

I adapted the regex so that the digest is optional.

By the way, if you don't want to do the entire work on your own at least check that what the LLM spat out is correct. replaceStringTemplate doesn't exist anymore / was renamed.

[acuntex]

@DerRockWolf I grabbed an older custom manager from my own repo as inspiration and shared my notes here to improve the user experience for folks configuring Renovate with CNPG.
If my comment came across as criticism, that wasn't my intent. Sorry about that. I appreciate you making the digest optional and pointing out the renamed key.
I value the project and want to contribute constructively. Tone matters for motivating contributors. My goal is collaboration, not contention.