Update the website getting started docs (#2203)

* Update the website getting started docs

Add a new example, fib, that contains an application for the computation
of Fibonacci numbers. Use this example to update the website getting
started documentation.

* Revise docs english

* Update example/fib/go.mod

Co-authored-by: Anthony Mirabella <a9@aneurysm9.com>

* Add a What's Next section

* Clean up intro

* Apply suggestions from code review

Co-authored-by: Anthony Mirabella <a9@aneurysm9.com>

* Apply feedback

* Return from Poll on error

* Update website_docs/getting-started.md

Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>

* Add root and parent relationship info

Co-authored-by: Anthony Mirabella <a9@aneurysm9.com>
Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>

example/fib/main.go

@@ -0,0 +1,101 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"context"
+	"io"
+	"log"
+	"os"
+	"os/signal"
+
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/exporters/stdout/stdouttrace"
+	"go.opentelemetry.io/otel/sdk/resource"
+	"go.opentelemetry.io/otel/sdk/trace"
+	semconv "go.opentelemetry.io/otel/semconv/v1.4.0"
+)
+
+// newExporter returns a console exporter.
+func newExporter(w io.Writer) (trace.SpanExporter, error) {
+	return stdouttrace.New(
+		stdouttrace.WithWriter(w),
+		// Use human readable output.
+		stdouttrace.WithPrettyPrint(),
+		// Do not print timestamps for the demo.
+		stdouttrace.WithoutTimestamps(),
+	)
+}
+
+// newResource returns a resource describing this application.
+func newResource() *resource.Resource {
+	r, _ := resource.Merge(
+		resource.Default(),
+		resource.NewWithAttributes(
+			semconv.SchemaURL,
+			semconv.ServiceNameKey.String("fib"),
+			semconv.ServiceVersionKey.String("v0.1.0"),
+			attribute.String("environment", "demo"),
+		),
+	)
+	return r
+}
+
+func main() {
+	l := log.New(os.Stdout, "", 0)
+
+	// Write telemetry data to a file.
+	f, err := os.Create("traces.txt")
+	if err != nil {
+		l.Fatal(err)
+	}
+	defer f.Close()
+
+	exp, err := newExporter(f)
+	if err != nil {
+		l.Fatal(err)
+	}
+
+	tp := trace.NewTracerProvider(
+		trace.WithBatcher(exp),
+		trace.WithResource(newResource()),
+	)
+	defer func() {
+		if err := tp.Shutdown(context.Background()); err != nil {
+			l.Fatal(err)
+		}
+	}()
+	otel.SetTracerProvider(tp)
+
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, os.Interrupt)
+
+	errCh := make(chan error)
+	app := NewApp(os.Stdin, l)
+	go func() {
+		errCh <- app.Run(context.Background())
+	}()
+
+	select {
+	case <-sigCh:
+		l.Println("\ngoodbye")
+		return
+	case err := <-errCh:
+		if err != nil {
+			l.Fatal(err)
+		}
+	}
+}