Canonicalise around the new "competition" simulator

For SR2021 we are running entirely in this simulator and don't
really want to have to update the old PyGame based simulator.
Given that the new simulator offers a much better experience anyway,
it seems much more reasonable to expect that we're going to use
it exclusively in the future.

Dropping the docs and links to the old simulator therefore reduces
the work we need to do now and avoids any potential confusion on
the part of the competitors.

This therefore also renames references to the simulator to simplify
things, but deliberately doesn't change:

- its position in the sidebar
  (its importance this year means it feels to me like it deserves
  to be both a top-level item and to be at the top of the list)
- the directories we mention that users will see when using it
  (changing this would involve changing the simulator repo itself,
  which I don't think is needed)

Author: PeterJCLaw

_data/sidebar_tree.yaml

@@ -3,13 +3,13 @@
 tree:
   - url: /
     title: Introduction
-  - url: /competition-simulator
-    title: Competition Simulator
+  - url: /simulator
+    title: Simulator
     tree:
-      - url: /competition-simulator/programming
+      - url: /simulator/programming
         title: Programming
         tree:
-          - url: /competition-simulator/programming/time
+          - url: /simulator/programming/time
             title: Time
   - url: /IDE/
     title: IDE
@@ -85,8 +85,6 @@ tree:
           title: Markers
     - url: /programming/git_repositories
       title: Git Repositories
-    - url: /programming/simulator
-      title: Simulator
   - url: /rules/
     title: Rules
     tree:

images/content/competition-simulator/arena.png renamed to images/content/simulator/arena.png

@@ -6,8 +6,8 @@ title: Included Python Libraries
 Included Libraries
 ==================
 
-Competition Simulator
----------------------
+Simulator
+---------
 
 The following libraries will be available to your robot code during the
 competition. Note that for local development you will need to install these

images/content/competition-simulator/robot-front.png renamed to images/content/simulator/robot-front.png

@@ -35,9 +35,9 @@ R = Robot()
 <div class="warning">
   <!-- We should drop this after SR2020 and move `ManualTimestepRobot` to being the default. -->
 
-  When programming for the <a href="/docs/competition-simulator">Competition Simulator</a>
+  When programming for the <a href="/docs/simulator">Competition Simulator</a>
   you are strongly encouraged to use the
-  <a href="/docs/competition-simulator/programming/time#manual-timestep-robot">
+  <a href="/docs/simulator/programming/time#manual-timestep-robot">
     <code>ManualTimestepRobot class</code>
   </a>
   instead of the <code>Robot</code> class. This avoids unpredictable behaviour