[Preview 7] Blazor directive updates

[guardrex]

• Drop '@' from values. Update blazor docs to reflect directive attributes changes #12756 (comment) and Clean up which directive attributes expect HTML vs C# aspnetcore#10909
• Drop @(...) syntax. Update blazor docs to reflect directive attributes changes #12756 (comment)

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Document Details

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• ID: be4c4d0c-149c-bde1-c7fa-f082916390ca
• Version Independent ID: 9d945996-ad02-bd4b-58ea-c465cc5cc74c
• Content: Create and use Razor components
• Content Source: aspnetcore/blazor/components.md
• Product: aspnet-core
• Technology: aspnetcore-blazor
• GitHub Login: @guardrex
• Microsoft Alias: riande