Docs: clarify conftest usage and fixture import pitfalls

[Dooopinder]

This PR clarifies fixture visibility vs scope, recommends conftest.py for sharing fixtures, and documents why importing fixtures is discouraged.

Clarifies that fixture scope controls lifetime and caching, not visibility

Explicitly explains that fixture visibility depends on where it is defined (test module, conftest.py, or plugin)

Recommends conftest.py as the correct and supported way to share fixtures across multiple test modules

Adds a clear warning against importing fixtures from test files or conftest.py, which can cause duplicate registration and confusing behavior

Adds a simple rule of thumb: never import fixtures except for type annotations

Links readers to the reference documentation for deeper technical details

closes #13148

[bluetech]

Thanks for looking at this. Please see my comments.

Also pinging @The-Compiler who might want to review this (the issue discusses a bit larger reworking of the docs around this IIUC).

[Dooopinder]

Thanks a lot for the detailed review! This makes sense — I’ll rework the text

[Dooopinder]

Hello @bluetech :) I’ve updated the text to integrate the notes into the main flow, clarified the wording, and addressed the type-annotation explanation. Please let me know if this looks better now.