Update developer-page.rst

[sderuiter]

Fixed link to CCDExplorer.io

Purpose

_Describe the purpose of the pull request, link to issue describing the problem, etc.

Changes

_Describe the changes that were needed.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

CLA acceptance

_Remove if not applicable.

By submitting the contribution I accept the terms and conditions of the
Contributor License Agreement v1.0

• link: https://developers.concordium.com/CLAs/Contributor-License-Agreement-v1.0.pdf

• I accept the above linked CLA.

[dragosgd]

This change looks good, thank you for your contribution!

[it09]

Hi Sderuiter, Great PR! Thanks so much