ci: replace docfx build from container to docfx command

[guitarrapc]

Description

The current docfx build using containers is failing (example build log), and this cannot be resolved from the user side. Let's change the documentation build process to use the docfx .NET tool instead.

doc generation was stopped for about 4 months, there are new APIs introduced, something like UnityAsyncExtensions.AssetBundleCreateRequestAwaiter.

Unity Assembly Considerations

UniTask is dependent on Unity, which means UnityEngine.dll and other Unity-related DLLs are required to build UniTask. However, since Unity assemblies are not distributed under OSS licenses, we cannot build the UniTask documentation by compiling from source as docfx normally would. Instead, we need to reference the pre-compiled UniTask DLLs.

see failed example. #676

When Unity loads the UniTask project (under src/UniTask), it compiles assemblies into UniTask/Library/ScriptAssemblies/UniTask*.dll. I have changed the docfx metadata source to use these assemblies, and API documentation is now generated successfully.

see Build log: https://github.com/Cysharp/UniTask/actions/runs/17609676115/job/50028467166

Notable Side Effects

• Cons: A Unity build is required to update GitHub Pages.
• Pros: Some APIs were missing when generating docfx documentation from source code, but referencing the compiled DLLs provides full API coverage.

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