Installation documentation is out of date

[masonwheeler]

Describe the bug
In windows-instructions.md, it says:

1. Run the following command inside of 'src/WebUI' to install the connection string

dotnet user-secrets set "ConnectionStrings:DefaultConnection" "YOUR_CONNECTION_STRING"

but the src/WebUI folder does not exist in the current source tree.

The same problem exists in the Linux setup documentation.

[asynts]

Oh, this should be src/WebApp, we renamed that recently.

Addendum. I am planning to rewrite the instructions entirely after #72 and codidact/authentication#10 are merged.

~20 days later: still hasn't happened, but I have this on my to do list.