Please update building documentation

We followed the building section of README.md and doc/building.md to build the terminal, and found several uncleared guidance.

Following the guidance, we expect:

1. git submodule command initialize 3rd party codebase
2. OpenConsole.sln restore successfully when launching the CascadiaPackage with the documented setup

While the actual behavior is:

1. .gitmodule present nowhere, thus the git command does nothing
2. Several nuget error with "Unable to find", such as
```
NU1101 Unable to find package Microsoft.AspNetCore.App.Ref. No packages exist with this id in source(s): C:\Program Files\dotnet\library-packs, TerminalDependencies TerminalStress G:\GithubRepos\terminal\src\tools\TerminalStress\TerminalStress.csproj 1
NU1101 Unable to find package Microsoft.NETCore.App.Ref. No packages exist with this id in source(s): C:\Program Files\dotnet\library-packs, TerminalDependencies TerminalStress G:\GithubRepos\terminal\src\tools\TerminalStress\TerminalStress.csproj 1
NU1101 Unable to find package Microsoft.WindowsDesktop.App.Ref. No packages exist with this id in source(s): C:\Program Files\dotnet\library-packs, TerminalDependencies TerminalStress G:\GithubRepos\terminal\src\tools\TerminalStress\TerminalStress.csproj 1
```
3. Several nuget restore error with site pkgs.dev.azure.com/shine-oss, reason is 401, Unauthorized

Please update the building documentation