Add documentation on creating custom endpoints

[glazychev-art]

Description

For various reasons, current standard applications may not meet user requirements. We need to provide a description and instructions how to create a custom endpoint

[316953425]

hi @glazychev-art @denis-tingaikin
I noticed that @d-uzlov has put the how to customize endpoint task in the In Progress state. I looked at the task status (https://github.com/orgs/networkservicemesh/projects/22/views/1) and I noticed The remaining time is only 0.5 hours. I guess you may have completed write the relevant documents. I would like to ask if the relevant introductory documents have been completed now. If so, can you send me the information? I'm very sorry to say I couldn't find the location of the relevant documents,
Our company appreciate the nsm project very much and I am like to learn it
As a first time user of Go and k8s, it is too difficult for me without documentation. I would be very grateful for any help you can give me.

[denis-tingaikin]

Hello @316953425

I've posted current version of this doc #261.

If you will have a time please have a look and let us know what things should be covered and we'll do improvements ;)

Preview: https://deploy-preview-266--nsm.netlify.app/docs/concepts/features/develop_nse

[316953425]

hi @denis-tingaikin
I think what you are doing is very important to the nsm community. It can help new people learn and understand nsm.
thanks a lot

My suggestion:
I think it might be easier for a newcomer to start with a simple example, such as (https://github.com/networkservicemesh/cmd-nse-icmp-responder/blob/main/main.go)
In the process of explaining this example, you can explain some core concepts in nsm
such as chain element mentioned in https://deploy-preview-266--nsm.netlify.app/docs/concepts/features/develop_nse and give examples.
Fox example：What does this code as followed mainly do?

	clientOptions := append(
		tracing.WithTracingDial(),
		grpc.WithBlock(),
		grpc.WithDefaultCallOptions(
			grpc.WaitForReady(true),
			grpc.PerRPCCredentials(token.NewPerRPCCredentials(spiffejwt.TokenGeneratorFunc(source, config.MaxTokenLifetime)))),
		grpc.WithTransportCredentials(
			grpcfd.TransportCredentials(
				credentials.NewTLS(
					tlsClientConfig,
				),
			),
		),
		grpcfd.WithChainStreamInterceptor(),
		grpcfd.WithChainUnaryInterceptor(),
	)

	if config.RegisterService {
		for _, serviceName := range config.ServiceNames {
			nsRegistryClient := registryclient.NewNetworkServiceRegistryClient(ctx,
				registryclient.WithClientURL(&config.ConnectTo),
				registryclient.WithDialOptions(clientOptions...),
				registryclient.WithAuthorizeNSRegistryClient(registryauthorize.NewNetworkServiceRegistryClient(
					registryauthorize.WithPolicies(config.RegistryClientPolicies...))))
			_, err = nsRegistryClient.Register(ctx, &registryapi.NetworkService{
				Name:    serviceName,
				Payload: config.Payload,
			})

			if err != nil {
				log.FromContext(ctx).Fatalf("unable to register ns %+v", err)
			}
		}
	}

	nseRegistryClient := registryclient.NewNetworkServiceEndpointRegistryClient(
		ctx,
		registryclient.WithClientURL(&config.ConnectTo),
		registryclient.WithDialOptions(clientOptions...),
		registryclient.WithNSEAdditionalFunctionality(
			clientinfo.NewNetworkServiceEndpointRegistryClient(),
			registrysendfd.NewNetworkServiceEndpointRegistryClient(),
		),
		registryclient.WithAuthorizeNSERegistryClient(registryauthorize.NewNetworkServiceEndpointRegistryClient(
			registryauthorize.WithPolicies(config.RegistryClientPolicies...))),
	)
	nse := getNseEndpoint(config, listenOn)
	nse, err = nseRegistryClient.Register(ctx, nse)
	logrus.Infof("nse: %+v", nse)

This is a document on how to develop the first module of nginx. I think this is a better way to get started. I hope you can refer to it. Thank you very much for your work.
https://tejgop.github.io/nginx-module-guide/#the-c-file