Add ASP.NET Core OpenAPI documentation to README

[Copilot]

Documents the OptionalValues.OpenApi package which provides support for ASP.NET Core's built-in OpenAPI (Microsoft.AspNetCore.OpenApi).

Changes

• Added OptionalValues.OpenApi to the packages table
• Added "ASP.NET Core OpenAPI" section under Library support
• Updated Features section to include ASP.NET Core OpenAPI
• Updated Table of Contents

Usage

builder.Services.AddOpenApi(options =>
{
    options.AddOptionalValueSupport();
});

Original prompt

Please write the documentation in the README.md for the ASP.NET Core OpenAPI support (implemented in OptionalValues.OpenApi).

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.