Update documentation to soften warnings about active development status

[tgilon]

Affected Component/Workflow

• I have labeled this feature with the affected component(s) using GitHub labels (e.g., SB, CBA, documentation, etc.).

Overview

We still have the old development status warning in the docs and Readme, and no link to the new project website.

Scope & Key Deliverables

Update the documentation and Readme now that we've successfully benchmarked TYNDP 2024

Suggestions were made in:

• Merge changes from upstream (incl. PyPSA-Eur v2026.02.0) #754 (comment)
• Merge changes from upstream (incl. PyPSA-Eur v2026.02.0) #754 (comment)

Implementation Tasks

• Link to website
• Soften warnings about active dev status