Resolve "[Docs] installation guide is misleading"

Summary

This MR aims to clarify that there are two concurrent methods of installation, of which developers need to choose the poetry one.

Explain your context

New developers shall clearly see what they need to do.

Please list breaking changes, new features or bugfixes

• Breaking changes:
• New features:
• Bugfixes: Improved installation docs

Closes #326 (closed)