Coverage can be merged with `dotnet-coverage` tool

[tillig]

Currently in the docs it directs folks trying to merge coverage reports to look at a comment from 2020 with some Powershell script to help merge coverage reports.

Since then, the dotnet-coverage tool has been released and allows you to merge coverage reports pretty easily.

For example, if your coverage reports are under artifacts/coverage and you have Cobertura format, you can use glob patterns to work around the randomly named directories VSTest creates and merge them like this:

dotnet-coverage merge artifacts/coverage/**/coverage.cobertura.xml -f cobertura -o artifacts/coverage/coverage.xml

It might be interesting to update the docs accordingly - this is far easier than the current suggestion.