Add diagnostics doc for ASPIREAZURE003

[Copilot]

ASPIREAZURE003 is the experimental diagnostic for Azure Virtual Network types in Aspire.Hosting.Azure.Network, introduced in 13.2 via dotnet/aspire#13108.

• New doc page diagnostics/aspireazure003.mdx following existing ASPIREAZURE001/002 pattern
• Added to overview table in overview.mdx
• Added to sidebar nav in reference.topics.ts

// Triggers ASPIREAZURE003
var vnet = builder.AddAzureVirtualNetwork("vnet");
var subnet = vnet.AddSubnet("pe-subnet", "10.0.1.0/24");

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