tutorial: add web service tutorial using Go and Gin

This adds an introductory tutorial on building a small RESTful API using Go
and the Gin web framework.

This change includes content on:

- Using the Gin web framework
  (https://pkg.go.dev/github.com/gin-gonic/gin).
- Creating an API that includes three HTTP method-and-endpoint
  combinations.
- GET and POST actions that convert between a Go struct and JSON.
- Supporting URL path parameters.
- Go command-line tools such as `go get` and `go run`.

This content is written in the enhanced Markdown format supported by the
interactive tutorial application in the Google Cloud Shell Editor.
For more, see https://cloud.google.com/shell/docs/cloud-shell-tutorials/tutorials

Change-Id: I216ec12763af83db257c978997a2987eae418995
Reviewed-on: https://go-review.googlesource.com/c/tour/+/328611
Trust: Steve Traut <straut@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

Author: Steve Traut