added example with http.NewServeMux

[halon176]

Describe the PR
I added an example that allows generating and displaying Swagger documentation using http.NewServeMux

Relation issue
e.g. https://github.com/swaggo/gin-swagger/pull/123/files

Additional context
The Go version must be 1.22 or higher to be able to use all the new features.

[ubogdan]

Did you test the r.Handle route ? Looks to me like invalid according to the documentation

import (
	"fmt"
	"net/http"
)

type apiHandler struct{}

func (apiHandler) ServeHTTP(http.ResponseWriter, *http.Request) {}

func main() {
	mux := http.NewServeMux()
	mux.Handle("/api/", apiHandler{})
	mux.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
		// The "/" pattern matches everything, so we need to check
		// that we're at the root here.
		if req.URL.Path != "/" {
			http.NotFound(w, req)
			return
		}
		fmt.Fprintf(w, "Welcome to the home page!")
	})
}

[ubogdan]

I want this example to work with (1.19 or newer).
What new features are you talking about?