Please update building documentation

[AlexeiNaabal]

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