Add multi-targeting section to trimming documentation

[Copilot]

Trimming is only supported in .NET 6+, but most libraries must support .NET Standard 2.0 or .NET Framework. The documentation lacked guidance on this essential multi-targeting requirement.

Changes

• New section: "Multi-targeting for trimming" added after "Enable library trim warnings"
  • Documents common multi-targeting patterns (netstandard2.0;net8.0 and net472;netstandard2.0;net8.0)
  • Shows conditional IsTrimmable setup using IsTargetFrameworkCompatible MSBuild function
  • Provides recommendations for library authors preparing trim-compatible libraries
  • Links to related breaking change documentation

Example

<PropertyGroup>
  <TargetFrameworks>netstandard2.0;net8.0</TargetFrameworks>
  <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
</PropertyGroup>

This enables IsTrimmable only for frameworks that support trimming (net6.0+) while avoiding warnings for .NET Standard and .NET Framework targets.

Fixes #36487

Original prompt

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This section details on the original issue you should resolve

<issue_title>Prepare .NET libraries for trimming: Add multi-targeting H2 with our recommendations</issue_title>
<issue_description>
See this comment copied below

Finally, recently we've learned that multi-targeting is not only very common, but basically a must have when enabling trimming. (This is based on experience trying to make some NuGets trim compatible). I think the document should have a section about the multi-targeting and our recommendations around it. But the core point is that very few libraries will only target latest TFM. By far the most common case is either library targets netstandard2.0 or it multi-targets net472, netstandard2.0 and net6.0 (or some combination of these).

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Document Details

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• ID: f3d1f980-933d-a2a8-1645-098c2a4bc2a2
• Version Independent ID: 80dc5a35-aee1-fa68-5a21-78c628e9ff41
• Content: Prepare .NET libraries for trimming - .NET
• Content Source: docs/core/deploying/trimming/prepare-libraries-for-trimming.md
• Product: dotnet-fundamentals
• GitHub Login: @sbomer
• Microsoft Alias: svbomer</issue_description>

Comments on the Issue (you are @copilot in this section)

@sbomer Leaving a note here to mention the new behavior in the `true` part of https://learn.microsoft.com/dotnet/core/deploying/trimming/prepare-libraries-for-trimming. @sbomer I'm adding a breaking change notice that mentions multi-targeting: https://github.com//pull/36775. We still need a dedicated section in the general trimming docs though. When that's added, we should also link to it from the breaking change notice.

• Fixes Prepare .NET libraries for trimming: Add multi-targeting H2 with our recommendations #36730

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