Update README with comprehensive documentation

[Copilot]

README was minimal—just a build badge, one-liner description, and spec links. Updated to match SuperSocket ecosystem standards and address user requests for examples (issue #19).

Changes

• Badges: Added NuGet version and download count
• Features: Listed key capabilities (MQTT 3.1.1, topic wildcards, .NET 6-9 support)
• Installation: NuGet commands for all three packages
• Quick Start: Working server and client examples
• Project Structure: Package descriptions table

Server Example

var host = SuperSocketHostBuilder
    .Create<MQTTPacket>()
    .UseMQTT()
    .UseInProcSessionContainer()
    .ConfigureAppConfiguration((hostCtx, configApp) =>
    {
        configApp.AddInMemoryCollection(new Dictionary<string, string?>
        {
            { "serverOptions:name", "MQTTServer" },
            { "serverOptions:listeners:0:ip", "Any" },
            { "serverOptions:listeners:0:port", "1883" }
        });
    })
    .Build();

await host.RunAsync();

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