docfx dotnet tool migration

Changes:

migrated from obsoleted docfx.console tool to new docfx dotnet tool with modern C# support
added build pipelines for PRs to test build passes before merge
downgraded image to 2019 temporary to be able to build .net Framework 4.5 projects (we plan to remove EOL .Net Framework versions support in linq2db 6)
removed IdentityServer4.LinqToDB project from API documentation as it doesn't build anymore and it blocks API doc generation
fixed LinqToDB.Identity branch reference
bump year to 2023

There are 30 xml-doc non-critical warnings (most of them in DynamicColumnInfo.cs) during build, which we will fix later alongside with warn-as-error build mode

I've compared generated HTML and don't see any issues with it:

a bit better HTML
more proper types rendering (e.g. NRT annotations and type aliases)
docs generation is much faster

Currently it doesn't build locally with

C:\Program Files\dotnet\sdk\7.0.200\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.GenerateAssemblyInfo.targets(209,5): error MSB4018: The "GetAssemblyVersion" task failed unexpectedly.
C:\Program Files\dotnet\sdk\7.0.200\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.GenerateAssemblyInfo.targets(209,5): error MSB4018: System.IO.FileLoadException: Could not load file or assembly 'NuGet.Versioning, Version=6.5.0.136, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. Could not find or load a specific file. (0x80131621)

so I've added build pipeline to PRs to test docs build before merge.

Could be same issue as https://github.com/dotnet/docfx/issues/8389#issuecomment-1443329593

nuget 6.5 release doesn't look healthy:

there is no console version release yet
VS/BuildTools ship 6.5.0.154 but error reference some unknown 6.5.0.136 version

linq2db / docs

docfx dotnet tool migration #42