_content/doc: add a Go/Gin web service tutorial

This adds a tutorial introducing how to build a RESTful web service
using Go and the Gin web framework.

This change includes:

- A RESTful web service tutorial:
  - _content/doc/tutorial/web-service-gin.md

- Updates to index topics so that they list the new content.
  - _content/doc/index.html
  - _content/doc/tutorial/index.html

Change-Id: I137dcd884c4e38b4a2bebd733ec630f2d9bc3b95
Reviewed-on: https://go-review.googlesource.com/c/website/+/332349
Trust: Steve Traut <straut@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

_content/doc/index.html

@@ -37,6 +37,12 @@ <h3 id="create-module-tutorial"><a href="/doc/tutorial/create-module.html">Tutor
 A tutorial of short topics introducing functions, error handling, arrays, maps, unit testing, and compiling.
 </p>
 
+<h3 id="web-service-gin-tutorial"><a href="/doc/tutorial/web-service-gin.html">Tutorial: Developing a RESTful
+    API with Go and Gin</a></h3>
+<p>
+Introduces the basics of writing a RESTful web service API with Go and the Gin Web Framework.
+</p>
+
 <h3 id="writing-web-applications"><a href="/doc/articles/wiki/">Writing Web Applications</a></h3>
 <p>
 Building a simple web application.

_content/doc/tutorial/index.html

@@ -40,14 +40,14 @@
         <td class="DocTable-cell">Introduces the basics of accessing a database
             using the standard library.</td>
       </tr>
-    <!-- <tr class="DocTable-row">
+    <tr class="DocTable-row">
       <td class="DocTable-cell">
         <a href="/doc/tutorial/web-service-gin">Developing a RESTful
             API with Go and Gin</a>
       </td>
       <td class="DocTable-cell">Introduces the basics of writing a RESTful web
 		service API with Go and the Gin Web Framework.</td>
-    </tr> -->
+    </tr>
     <tr class="DocTable-row">
       <td class="DocTable-cell">
         <a href="https://tour.golang.org/welcome/1">A Tour of Go</a>