Provide docs on how to integrate custom msbuild target with dotnet watch and hot reload

[Liero]

Is your feature request related to a problem? Please describe.

I want my app to do all the compilation in watch mode, when either user start the project from Visual Studio using F5, or from CLI using dotnet watch run.

I don't want to invoke multiple processes from CLI like:

Start-Job { dotnet watch run }; # watch C# changes
Start-Job { dotnet watch /t:YarnBuild }; # watch javascript changes

I have read quite some MS Build and .NET docs recently, but I'm missing more detailed info on how dotnet watch (run/build) works with msbuild.

For example:

1. how to start custom watch process when when dotnet watch run is invoked, simmilar to Microsoft.TypeScript.MSBuild.

   For example, I have SASS compiler cli `` or custom dotnet tool, that supports watch mode, and I would like it to run together with my dotnet watch run process, so that my SASS files are being compiled incrementally.

2. how to invoke specific msbuild target when dotnet watch run first builds the project, but not when it does hot reload in watch mode

Additional context

It has been asked couple of times, e.g.:

https://stackoverflow.com/questions/51824979/defining-command-to-be-run-with-dotnet-watch

https://stackoverflow.com/questions/70259508/how-to-integrate-a-watch-process-into-ms-build-and-dotnet-watch-run

It's also a missing part of otherwise nice Microsoft.AspNetCore.ClientAssets package, see
https://devblogs.microsoft.com/dotnet/build-client-web-assets-for-your-razor-class-library/#comment-14441

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[ilyvion]

☝️ When "keeping your backlog clean" is more important than addressing issues people bring up, apparently.

[Liero]

☝️ When "keeping your backlog clean" is more important than addressing issues people bring up, apparently.

Especially when it has been assigned and added to "Discussion" milestone. This one hurts all the more so, because it would help the community to create missing tooling, so the impact could be significant