Improve doc: make "using-thymeleaf" clickable

doc/tutorials/2.1/thymeleafspring.html

@@ -144,7 +144,7 @@ <h1>1 Integrating Thymeleaf with Spring</h1>
 <li>Create forms in your templates that are completely integrated with your form-backing beans and result bindings, including the use of property editors, conversion services and validation error handling.</li>
 <li>Display internationalization messages from messages files managed by Spring (through the usual <code>MessageSource</code> objects).</li>
 </ul>
-<p>Note that in order to fully understand this tutorial, you should have first gone through the <em>“Using Thymeleaf”</em> tutorial, which explains the Standard Dialect in depth.</p>
+<p>Note that in order to fully understand this tutorial, you should have first gone through the <a href="https://www.thymeleaf.org/documentation.html#using-thymeleaf"><em>“Using Thymeleaf”</em></a> tutorial, which explains the Standard Dialect in depth.</p>
 </section>
 <section id="the-springstandard-dialect" class="level1">
 <h1>2 The SpringStandard Dialect</h1>

doc/tutorials/3.0/thymeleafspring.html

@@ -142,7 +142,7 @@ <h1>1 Integrating Thymeleaf with Spring</h1>
 <li>Display internationalization messages from message files managed by Spring (through the usual <code>MessageSource</code> objects).</li>
 <li>Resolve your templates using Spring’s own resource resolution mechanisms.</li>
 </ul>
-<p>Note that in order to fully understand this tutorial, you should have first gone through the <em>“Using Thymeleaf”</em> tutorial, which explains the Standard Dialect in depth.</p>
+<p>Note that in order to fully understand this tutorial, you should have first gone through the <a href="https://www.thymeleaf.org/documentation.html#using-thymeleaf"><em>“Using Thymeleaf”</em></a> tutorial, which explains the Standard Dialect in depth.</p>
 </section>
 <section id="the-springstandard-dialect" class="level1">
 <h1>2 The SpringStandard Dialect</h1>

doc/tutorials/3.1/thymeleafspring.html

@@ -152,7 +152,7 @@ <h1>1 Integrating Thymeleaf with Spring</h1>
 <li>Display internationalization messages from message files managed by Spring (through the usual <code>MessageSource</code> objects).</li>
 <li>Resolve your templates using Spring’s own resource resolution mechanisms.</li>
 </ul>
-<p>Note that in order to fully understand this tutorial, you should have first gone through the <em>“Using Thymeleaf”</em> tutorial, which explains the Standard Dialect in depth.</p>
+<p>Note that in order to fully understand this tutorial, you should have first gone through the <a href="https://www.thymeleaf.org/documentation.html#using-thymeleaf"><em>“Using Thymeleaf”</em></a> tutorial, which explains the Standard Dialect in depth.</p>
 </section>
 <section id="the-springstandard-dialect" class="level1">
 <h1>2 The SpringStandard Dialect</h1>