Clarify documention on default shell in windows

What does this MR do?

Clarify documention on default shell in windows

Why was this MR needed?

The documentation in docs/shells/_index.md states that pwsh is the default shell on Windows. This is not true, it is only the default at registration time. At runtime, the docker-windows and kubernetes executor hardcodes powershell (Desktop) via setupDefaultExecutorOptions in executors/docker/docker.go:

e.ExecutorOptions.Shell.Shell = shells.SNPowershell

In kuberntes its the same issue in setupDefaultExecutorOptions executors/kubernetes/kubernetes :

s.ExecutorOptions.Shell.Shell = shells.SNPowershell

so a user reading the docs would expect pwsh to run their jobs, but powershell is what actually executes them.

What are the relevant issue numbers?

#39377 (closed)