[BUGFIX] Mention that site config is auto generated for root pages (#715)

Updates the "Site Management" page with an attention admonition informing about the automated generation of page configurations when you create a page in the root of the page tree. Also adds another section header about editing the site configuration in order to keep creating and editing the site configuration as separate steps.

Author: mabolek

Documentation/FirstProject/SiteManagement/Index.rst

@@ -15,6 +15,12 @@ Site Management
 Create a new site configuration
 ===============================
 
+..  attention::
+    You can skip this step if you have already :ref:`created the root page <t3start:create-root-page>` 
+    in the root of the page tree. This will automatically generate a site 
+    configuration named after your page. Click the name of the page in the
+    list of site configurations to :ref:`edit the site configuration <t3start:edit-site-configuration>`.
+
 Before creating a site configuration, you should have :ref:`created the root page <t3start:create-root-page>` of
 your site in the page tree.
 
@@ -31,7 +37,13 @@ page to create a site configuration for it.
 
     Create a new site configuration
 
-In the next step, you can enter some basic information about the site.
+
+..  _edit-site-configuration:
+
+Edit the site configuration
+===========================
+
+You can now enter some basic information about the site.
 
 The site identifier can be arbitrary, so we use "example_site" here. The
 entry point should be a URL pointing to the web root. In this example, it is