#3469 Docs: add PowerShell to Set Up Shell Completions

swamp-club#3459 adds swamp completions powershell. content/manual/how-to/set-up-shell-completions.md covers bash, zsh and fish only. Add a PowerShell section: append the line swamp completions powershell | Out-String | Invoke-Expression to the PowerShell profile (the file named by the PROFILE automatic variable, created if missing), then restart the shell. Note that it works in Windows PowerShell 5.1 and PowerShell 7, and that model and workflow names complete from the current directory like the other shells.