[Static web assets] Add section detailing third-party integration with static web assets

[javiercn]

TL;DR

Items must be declared in the Content item group before the AssignTargetPaths target.

For integrating with JS build processes, reference the client assets package.

More details can be found https://devblogs.microsoft.com/dotnet/build-client-web-assets-for-your-razor-class-library/

This is .NET 6.0 and onwards.

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Document Details

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• ID: 9fd019c7-ceb7-6ef3-26ba-6b44341b6fc5
• Version Independent ID: fd116d00-0dc7-efaf-c714-34d77b345a05
• Content: Reusable Razor UI in class libraries with ASP.NET Core
• Content Source: aspnetcore/razor-pages/ui-class.md
• Product: aspnet-core
• Technology: aspnetcore-razorpages
• GitHub Login: @Rick-Anderson
• Microsoft Alias: riande

[SteveSandersonMS]

Since the issue description is fairly ad-hoc, just want to make sure the original goal is preserved:

We need to decide which parts of SWA are public, supported APIs (e.g., which MSBuild tasks, properties, metadata, enum values, etc.) and ensure they are fully documented and remain consistent in behavior across versions.