docs: document Native AOT guidance and fix [PromptBindable] code examples (#412)

[mkarimdev]

Fixes #412

Summary of Changes

1. Fixed Prompt.Bind Documentation Examples:

   • Added using Sharprompt; and [PromptBindable] attributes across model examples in README.md, docs/en/prompt-types/bind.md, and docs/ja/prompt-types/bind.md to resolve runtime InvalidOperationException issues for new users.
   • Documented model requirements (class type, public parameterless constructor, accessible setters).
   • Documented [MemberItems] usage and CancellationToken support in Prompt.Bind.

2. Documented Native AOT & Roslyn Source Generators:

   • Added a dedicated "Native AOT & Source Generators" section in docs/en/advanced.md and docs/ja/advanced.md.
   • Documented project configuration (<PublishAot>true</PublishAot>) and publish CLI commands.
   • Clarified enum metadata handling (source-generated module initializers for local enums vs. reflection fallback/EnumMetadataRegistry.Register<TEnum>() for external assembly enums).
   • Documented TypeConverter behavior under trimming.

3. Sample Alignment:

   • Updated samples/Sharprompt.Example/Program.cs with a source-generated JsonSerializerContext (SampleJsonSerializerContext) for MyFormModel to ensure trim/AOT warning-free execution.

Verification

• dotnet build -c Release (0 Warnings, 0 Errors)
• dotnet test -c Release --no-build -f net10.0 (351 Passed)
• dotnet format --verify-no-changes (0 formatting issues)
• npm run docs:build (VitePress site compiled with 0 errors)