CompatibilitySuppressions.xml file format is not fully documented

The article describes the basic structure of the _CompatibilitySuppressions.xml_ file but leaves out a lot of interesting details.
Since there is no tooling or IDE integration for this, it must be read and understood manually to assess the breaking changes made in a PR or in an upcoming release.

For example:
- The rules itself are not documented (similar to https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules). It is not even clear what rules exist.
- Depending on the rule, ``, ``, `` may contain different types of content or the element can be omitted altogether (e.g. [PKV0001](https://github.com/dotnet/aspnetcore/blob/17993688eff8a3fc91a91985713ecd3c482a6ce7/src/Framework/App.Runtime/src/CompatibilitySuppressions.xml#L4) vs. [CP1002](https://github.com/dotnet/aspnetcore/blob/17993688eff8a3fc91a91985713ecd3c482a6ce7/src/Caching/SqlServer/src/CompatibilitySuppressions.xml#L4) vs. [CP0002](https://github.com/dotnet/ClangSharp/blob/32bbe8a857ce8e723557ba50dc78b4126bf9717b/sources/ClangSharp/CompatibilitySuppressions.xml#L5)). How must valid content look like?

> Just a side note: That file might not really be review-friendly for humans because it is a bit hard to parse and the file format is not fully documented.

_Originally posted by @bitbonk in https://github.com/dotnet/docs/pull/36954#discussion_r1313887454_

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* Content: [.NET Package Validation - .NET](https://learn.microsoft.com/en-us/dotnet/fundamentals/package-validation/overview)
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