docs: Update outdated Helm repository URL in README

[pierreneter]

Description

The README documentation for Checkpointz currently points to an outdated and unreachable Helm repository URL (https://skylenet.github.io/ethereum-helm-charts). This causes a 404 Not Found error when users attempt to add the repository.

The repository has been migrated to the official ethPandaOps GitHub Pages URL: https://ethpandaops.github.io/ethereum-helm-charts.

Steps to Reproduce

1. Run the command listed in the README:

# Add the outdated ethereum-helm-charts repository
helm repo add ethereum-helm-charts https://skylenet.github.io/ethereum-helm-charts

2. Actual Behavior
   The terminal returns the following error:

Error: looks like "https://skylenet.github.io/ethereum-helm-charts" is not a valid chart repository or cannot be reached: failed to fetch https://skylenet.github.io/ethereum-helm-charts/index.yaml : 404 Not Found

3. Expected Behavior
   The documentation should be updated to point to the correct and active Helm repository URL:

# Add the correct ethereum-helm-charts repository
helm repo add ethereum-helm-charts https://ethpandaops.github.io/ethereum-helm-charts

[beanscg]

Verified this against current master:

• Source: README.md:208 still links the Helm chart source to https://github.com/skylenet/ethereum-helm-charts/tree/master/charts/checkpointz.
• Source: README.md:210 still uses helm repo add ethereum-helm-charts https://skylenet.github.io/ethereum-helm-charts.
• Current behavior: https://skylenet.github.io/ethereum-helm-charts/index.yaml returns HTTP 404.
• Current live repo index: https://ethpandaops.github.io/ethereum-helm-charts/index.yaml returns HTTP 200.
• Duplicate check: the open PRs I saw are unrelated dependency/runtime changes and do not appear to cover this Helm repo URL.

Smallest fix looks like switching the README Helm repository URL from skylenet.github.io to ethpandaops.github.io.