Restore Custom Favicon section in the reference guide

The section about favicon was mistakenly removed in 5fceb9d and this
commit reinstates it.

See gh-31224

spring-boot-project/spring-boot-docs/src/docs/asciidoc/anchor-rewrite.properties

@@ -773,6 +773,7 @@ features.developing-web-applications.spring-mvc.json=web.servlet.spring-mvc.json
 features.developing-web-applications.spring-mvc.message-codes=web.servlet.spring-mvc.message-codes
 features.developing-web-applications.spring-mvc.static-content=web.servlet.spring-mvc.static-content
 features.developing-web-applications.spring-mvc.welcome-page=web.servlet.spring-mvc.welcome-page
+features.developing-web-applications.spring-mvc.favicon=web.servlet.spring-mvc.favicon
 features.developing-web-applications.spring-mvc.content-negotiation=web.servlet.spring-mvc.content-negotiation
 features.developing-web-applications.spring-mvc.binding-initializer=web.servlet.spring-mvc.binding-initializer
 features.developing-web-applications.spring-mvc.template-engines=web.servlet.spring-mvc.template-engines

spring-boot-project/spring-boot-docs/src/docs/asciidoc/web/servlet.adoc

@@ -216,6 +216,13 @@ If either is found, it is automatically used as the welcome page of the applicat
 
 
 
+[[web.servlet.spring-mvc.favicon]]
+==== Custom Favicon
+As with other static resources, Spring Boot checks for a `favicon.ico` in the configured static content locations.
+If such a file is present, it is automatically used as the favicon of the application.
+
+
+
 [[web.servlet.spring-mvc.content-negotiation]]
 ==== Path Matching and Content Negotiation
 Spring MVC can map incoming HTTP requests to handlers by looking at the request path and matching it to the mappings defined in your application (for example, `@GetMapping` annotations on Controller methods).