Further opt-in trimming guidance (class libs)

[guardrex]

Fixes #36126

cc: @hakenr

Let's see about filling in the additional detail following the work in #36094:

• Move the trimming granularity section up to facilitate the Configuration section cross-linking to it with more information.
• Explain the use of <IsTrimmable> in the Configuration section.
• Cross-link the "explicitly opted in" remark of the Custom type section to the Configuration section, and add "class libraries" to it to clarify what is being opted-in.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/host-and-deploy/configure-trimmer.md	aspnetcore/blazor/host-and-deploy/configure-trimmer

[Copilot]

Pull Request Overview

This PR enhances the trimming guidance for ASP.NET Core Blazor applications by reorganizing content and adding more specific information about class library opt-in behavior. The changes improve the documentation's structure and provide clearer guidance on configuring trimming for class libraries.

Key changes:

• Reorganizes content by moving the trimming granularity section earlier in the document
• Adds detailed explanation of the <IsTrimmable> property for class library trimming opt-in
• Improves cross-references between sections to better connect related concepts

[danroth27]

A few more suggestions to discuss.

[guardrex]

@danroth27 ... I made your suggested changes with only minor style tweaks.

[danroth27]

Looks good. One minor comment.