[Feature]: Standardizes the shell mount commands for PowerShell 7 in the documentation

[linghengqian]

1. Your usage scenarios?

• Starting from the merge at Simplify PowerShell profile command in README #521, the command to mount the shell in PowerShell 7 has become:

echo 'Invoke-Expression "$(vfox activate pwsh)"' >> $PROFILE

• However, much of the documentation still uses if (-not (Test-Path -Path $PROFILE)) { New-Item -Type File -Path $PROFILE -Force }; Add-Content -Path $PROFILE -Value 'Invoke-Expression "$(vfox activate pwsh)"'. See https://vfox.dev/guides/quick-start.html#_2-hook-vfox-to-your-shell .
• 

2. What is your expected outcome?

• Standardizes the shell mount commands for PowerShell 7 in the documentation. All we need to do is if (-not (Test-Path -Path $PROFILE)) { New-Item -Type File -Path $PROFILE -Force }; Add-Content -Path $PROFILE -Value 'Invoke-Expression "$(vfox activate pwsh)"'.
• The command echo 'Invoke-Expression "$(vfox activate pwsh)"' >> $PROFILE is meaningless because the $PROFILE file does not exist by default.