chore: add load-on-startup configuration for custom Vaadin servlet

As of Flow 23.2, Vite dev-server needs VaadinServlet to be initialized before
to start. In order to have Vite starting eagerly (before any HTTP request),
VaadinServlet must be registered with load-on-startup attribute to a value
greater than zero. For custom VaadinServlet this must be done explicitly by
the developer.
This change updates example code defining custom Vaadin Servlet by adding
load-on-startup setting.

Part of vaadin/flow#14479

articles/advanced/i18n-localization.asciidoc

@@ -26,8 +26,8 @@ mvn jetty:run -Dvaadin.i18n.provider=com.vaadin.example.ui.TranslationProvider
 When using the annotation, you could have the servlet class as:
 [source,java]
 ----
-@WebServlet(urlPatterns = "/*", name = "slot", asyncSupported = true, initParams = {
-        @WebInitParam(name = Constants.I18N_PROVIDER, value = "com.vaadin.example.ui.TranslationProvider") })
+@WebServlet(urlPatterns = "/*", name = "slot", asyncSupported = true, loadOnStartup = 1, 
+   initParams = { @WebInitParam(name = Constants.I18N_PROVIDER, value = "com.vaadin.example.ui.TranslationProvider") })
 public class ApplicationServlet extends VaadinServlet {
 }
 ----
@@ -49,6 +49,7 @@ Or, using the [filename]#web.xml# file:
     <servlet-class>
         com.vaadin.flow.server.VaadinServlet
     </servlet-class>
+    <load-on-startup>1</load-on-startup>
 
     <init-param>
       <param-name>i18n.provider</param-name>

articles/configuration/npm-pnpm.asciidoc

@@ -65,8 +65,8 @@ Alternatively, you can use the Servlet 3.0 `@WebServlet` annotation:
 
 [source,java]
 ----
-@WebServlet(urlPatterns = "/*", name = "myservlet", asyncSupported = true, initParams = {
-        @WebInitParam(name = "pnpm.enable", value = "true") })
+@WebServlet(urlPatterns = "/*", name = "myservlet", asyncSupported = true, loadOnStartup = 1,
+  initParams = { @WebInitParam(name = "pnpm.enable", value = "true") })
 public class CustomServlet extends VaadinServlet {
 }
 ----
@@ -88,6 +88,7 @@ or use the traditional [filename]#web.xml# file:
     <servlet-class>
         com.vaadin.flow.server.VaadinServlet
     </servlet-class>
+    <load-on-startup>1</load-on-startup>
 
     <init-param>
       <param-name>pnpm.enable</param-name>

articles/configuration/properties.asciidoc

@@ -49,7 +49,8 @@ You can use the Servlet 3.0 `@WebServlet` annotation, which requires you to conf
 
 [source,java]
 ----
-@WebServlet(urlPatterns = "/*", name = "myservlet", asyncSupported = true, initParams = {
+@WebServlet(urlPatterns = "/*", name = "myservlet", asyncSupported = true, , loadOnStartup = 1,
+    initParams = {
         @WebInitParam(name = "frontend.url.es6", value = "http://mydomain.com/es6/"),
         @WebInitParam(name = "frontend.url.es5", value = "http://mydomain.com/es5/") })
 public class MyServlet extends VaadinServlet {
@@ -73,6 +74,7 @@ Yet another approach is to use the [filename]#web.xml# file:
     <servlet-class>
         com.vaadin.flow.server.VaadinServlet
     </servlet-class>
+    <load-on-startup>1</load-on-startup>
 
     <init-param>
       <param-name>frontend.url.es6</param-name>

articles/integrations/embedding/exporter.asciidoc

@@ -166,7 +166,7 @@ public final class UserService {
 
 [source,java]
 ----
-@WebServlet(urlPatterns = { "/vaadin/*" })
+@WebServlet(urlPatterns = { "/vaadin/*" }, asyncSupported = true, loadOnStartup = 1)
 public class WebComponentVaadinServlet extends VaadinServlet {
 }
 ----