doc: remove builtin `sl` in windows powershell

[huhuang03]

https://github.com/facebook/sapling/blob/main/website/docs/introduction/installation.md

Note that the name of the Sapling CLI sl.exe conflicts with the sl shell built-in in PowerShell (sl is an alias for Set-Location, which is equivalent to cd). If you want to use sl to run sl.exe in PowerShell, you must reassign the alias. Again, you must run the following as Administrator:

Set-Alias -Name sl -Value 'C:\Program Files\Sapling\sl.exe' -Force -Option Constant,ReadOnly,AllScope{'\n'} sl --version{'\n'} Sapling {latestReleaseVersion}

Set-Alias -Name sl -Value can't overwrite sl on my computer, still Set-Location, but Remove-Item Alias:sl -Force does