Add old oep-code documentation to new docs

[jh-RLI]

Summary of the discussion

We want to ease the creation of documentation by writing google-style docstrings and markdown. Also, we reduce the 3rd party deps. by stepping back from ReadTheDocs platform.

Type of change (CHANGELOG.md)

Added

• Rework the Documentation and introduce mkdocs + material + mkdocstrings (#)

Workflow checklist

Automation

Closes #1277

PR-Assignee

• [x] 🐙 Follow the workflow in CONTRIBUTING.md
• [ ] 📝 Update the CHANGELOG.md
• [ ] 📙 Update the documentation on Read The Docs

Reviewer

• [ ] 🐙 Follow the Reviewer Guidelines
• [ ] 🐙 Provided feedback and show sufficient appreciation for the work done