Update class libraries doc

Improvements:

- Doesn't go into detail when/why you should create class libraries. The first paragraph seems to be it. Same with patterns (when should you go from 1 library to 2.
- Makes it seem like you need to choose between four types of class libraries. In reality, there are actually five types, and the fifth one is the one you should be using and only consider the others as advanced scenarios.
- Doesn't describe how to create class libraries.
- Doesn't mention the truly interesting library techniques like partial.
- We need to stop pushing ".NET Assembly file format" as first-paragraph level of content. If you interviewed 100 people, well less than 10 of them would want to better understand the file format.
- Add a link to https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/.

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#### Document Details

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* ID: d65cec7b-798d-9bf3-be94-f2bfbc9182aa
* Version Independent ID: 8e78e5d4-0b17-1f69-c091-05eb3e52aabe
* Content: [.NET class libraries](https://docs.microsoft.com/en-us/dotnet/standard/class-libraries)
* Content Source: [docs/standard/class-libraries.md](https://github.com/dotnet/docs/blob/main/docs/standard/class-libraries.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @richlander
* Microsoft Alias: **gewarren**

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[Associated WorkItem - 57953](https://dev.azure.com/msft-skilling/Content/_workitems/edit/57953)