Add brief highlevel docs on cookie / session

docs/en/02_Developer_Guides/18_Cookies_And_Sessions/01_Cookies.md

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+title: Cookies
+summary: A set of static methods for manipulating PHP cookies.
+
+# Cookies
+
+Cookies are a mechanism for storing data in the remote browser and thus tracking or identifying return users. 
+SilverStripe uses cookies for remembering users preferences. Application code can modify a users cookies through
+the [api:Cookie] class. This class mostly follows the PHP API.
+
+## set
+
+Sets the value of cookie with configuration.
+
+	:::php
+	Cookie::set($name, $value, $expiry = 90, $path = null, $domain = null, $secure = false, $httpOnly = false);
+
+	// Cookie::set('MyApplicationPreference', 'Yes');
+
+## get
+
+Returns the value of cookie.
+
+	:::php
+	Cookie::get($name);
+
+	// Cookie::get('MyApplicationPreference');
+	// returns 'Yes'
+
+## force_expiry
+
+Clears a given cookie.
+
+	:::php
+	Cookie::force_expiry($name, $path = null, $domain = null);
+
+	// Cookie::force_expiry('MyApplicationPreference')
+
+## API Documentation
+
+* [api:Cookie]

docs/en/02_Developer_Guides/18_Cookies_And_Sessions/02_Sessions.md

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+title: Sessions
+summary: A set of static methods for manipulating PHP sessions.
+
+# Sessions
+
+Session support in PHP consists of a way to preserve certain data across subsequent accesses such as logged in user
+information and security tokens.
+
+In order to support things like testing, the session is associated with a particular Controller.  In normal usage,
+this is loaded from and saved to the regular PHP session, but for things like static-page-generation and
+unit-testing, you can create multiple Controllers, each with their own session.
+ 
+## set
+
+	:::php
+	Session::set('MyValue', 6);
+
+Saves the value of to session data. You can also save arrays or serialized objects in session (but note there may be 
+size restrictions as to how much you can save).
+
+	:::php
+	// saves an array
+	Session::set('MyArrayOfValues', array('1','2','3'));
+
+	// saves an object (you'll have to unserialize it back)
+	$object = new Object();
+	Session::set('MyObject', serialize($object));
+ 
+## get
+
+Once you have saved a value to the Session you can access it by using the `get` function. Like the `set` function you 
+can use this anywhere in your PHP files.
+
+	:::php
+	echo Session::get('MyValue'); 
+	// returns 6
+
+	$data = Session::get('MyArrayOfValues'); 
+	// $data = array(1,2,3)
+
+	$object = unserialize(Session::get('MyObject', $object)); 
+	// $object = Object()
+
+## get_all
+
+You can also get all the values in the session at once. This is useful for debugging.
+	
+	:::php
+	Session::get_all(); 
+	// returns an array of all the session values.
+
+## clear
+
+Once you have accessed a value from the Session it doesn't automatically wipe the value from the Session, you have
+to specifically remove it. 
+
+	:::php
+	Session::clear('MyValue');
+
+Or you can clear every single value in the session at once. Note SilverStripe stores some of its own session data
+including form and page comment information. None of this is vital but `clear_all` will clear everything.
+	
+	:::php
+	Session::clear_all();
+
+## API Documentation
+
+* [api:Session]

docs/en/02_Developer_Guides/18_Cookies_And_Sessions/index.md

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+title: Cookies and Sessions
+summary: Save state information using the Cookie class and the Session class.
+introduction: Both the Cookie and Session classes can be used to preserve certain data across subsequent page requests.
+
+[CHILDREN]
+
+## API Documentation
+
+* [api:Cookie]
+* [api:Session]