Add eGon to open data tool section

[jh-RLI]

Description of the issue

We intorduced the "Open Data Tools" section in the nav bar. The eGon ecosystm is using the OEP as data arcive and recent developments made the data publishing of tools like egon-data more integrated with the OEP. That also contributed to OEF tools lke OMI.

Ideas of solution

I would like to add the eGon website to that list:

https://rego-n.org/

Workflow checklist

• I am aware of the workflow in
  CONTRIBUTING.md