Add non Spring Boot Spring Web MVC demo for springdoc (#67)

[DongJu-Na]

This PR adds a new demo module: demo-spring-webmvc-nonboot.

It addresses issue #67 by providing a working sample for using
springdoc-openapi in a classic Spring Web MVC application without
a Spring Boot application class.

Highlights

• New module demo-spring-webmvc-nonboot (WAR packaging)
• DispatcherServlet configured via web.xml
• Java config OpenApiConfiguration:
  • enables @EnableWebMvc
  • imports SpringDocConfiguration, SpringDocWebMvcConfiguration,
    SwaggerConfig and JacksonAutoConfiguration
  • explicitly registers a SpringDocConfigProperties bean
• Sample HelloController exposing GET /api/hello
• Module-level README describing how to build and deploy the demo

The demo was verified locally by deploying the generated WAR
to a Servlet container and accessing:

• /v3/api-docs
• /swagger-ui/index.html
• /api/hello

Fixes #67.