Some docs are duplicated here and in dotnet/docs

Docs like [error-codes.md](https://github.com/dotnet/linker/blob/main/docs/error-codes.md), [fixing-warnings.md](https://github.com/dotnet/linker/blob/main/docs/fixing-warnings.md), and other usage docs are duplicated in dotnet/docs or should be moved there and removed from this repo.

- [ ] [custom-steps.md](https://github.com/dotnet/linker/blob/main/docs/custom-steps.md)
- [ ] [error-codes.md](https://github.com/dotnet/linker/blob/main/docs/error-codes.md)
- [ ] [fixing-warnings.md](https://github.com/dotnet/linker/blob/main/docs/fixing-warnings.md)
- [ ] [illink-options.md/trimming-options.md](https://github.com/dotnet/linker/blob/main/docs/illink-options.md)
- [ ] [illink-tasks.md](https://github.com/dotnet/linker/blob/main/docs/illink-tasks.md)
- [ ] [serialization.md](https://github.com/dotnet/linker/blob/main/docs/serialization.md)