[Docs] Add migration guide for GraphQL.NET

[tobias-tengler]

No description provided.

[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the tte/graphql-dotnet-m... branch remains at 49%, unchanged from the main branch.

Show a code coverage summary of the most impacted files.

File	main 0dbeaa9	tte/graphql-dotnet-m... 48e7381	+/-
/home/runner/wo...ageProcessor.cs	100%	89%	-11%
/home/runner/wo...tocolHandler.cs	85%	85%	0%
/home/runner/wo.../MemoryArena.cs	81%	82%	+1%
/home/runner/wo...nManagerBase.cs	82%	83%	+1%
/home/runner/wo...ddlewareBase.cs	66%	67%	+1%
/home/runner/wo...DefaultTopic.cs	88%	89%	+1%
/home/runner/wo...tionRegistry.cs	86%	88%	+2%
/home/runner/wo...gerMessage.g.cs	6%	10%	+4%
/home/runner/wo...eiveEndpoint.cs	72%	77%	+5%
/home/runner/wo...tionTestBase.cs	72%	80%	+8%

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