Update bad shell assumptions from "Shell autocompletion" page (#11202)
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## Summary
- PowerShell isn't Windows-only
- Bash/Elvish isn't UNIX-only
- PowerShell profile won't necessarily exist
- Extracted the `echo $SHELL` tip to a tip tooltip
The new PowerShell lines are taken directly from
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_profiles?view=powershell-7.5#how-to-create-a-profile
Note that the "Standalone installer" commands are unaffected. Running
the Linux one through PowerShell worked just fine.
## Test Plan
Look at the page generated from this PR
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uv
b83e25a9 - Update bad shell assumptions from "Shell autocompletion" page (#11202)
