Remove various unused images (#11884)

Removes a few unused images, renames a few others for consistency, and
adds some documents explaining the existence of others not used by the
site.

Contributes to #11883

Author: parlough

src/content/assets/images/docs/android/build-variant-menu.png

@@ -0,0 +1,2 @@
+These images aren't used on our site,
+but they're referenced elsewhere, so shouldn't be deleted for now.

src/content/assets/images/docs/android/gradle-script-menu.png

@@ -0,0 +1,2 @@
+The `diagram-layercake.png` image isn't used on our site,
+but it's referenced elsewhere, so shouldn't be deleted for now.

src/content/assets/images/docs/arch-overview/web-framework-diagram.drawio.png renamed to src/content/assets/images/docs/arch-overview/web-framework-diagram.png

@@ -14,6 +14,7 @@ The web is just another device target for your app.
 To get started, visit [Building a web application with Flutter][].
 
 ## Powered by WebAssembly
+
 Dart and Flutter can compile to WebAssembly,
 a binary instruction format that enables fast apps on all major browsers.
 
@@ -23,6 +24,7 @@ check out the following video.
 {% ytEmbed 'lpnKWK-KEYs?start=1712', 'What\'s new in Flutter - WebAssembly' %}
 
 ## How it works
+
 Adding web support to Flutter involved implementing Flutter's
 core drawing layer on top of standard browser APIs, in addition
 to compiling Dart to JavaScript, instead of the ARM machine code that
@@ -35,11 +37,10 @@ Flutter core and framework along with your application
 into a single, minified source file that can be deployed to
 any web server.
 
-<img src="/assets/images/docs/arch-overview/web-framework-diagram.drawio.png"
-alt="Flutter architecture for web"
-width="100%">
+<img src="/assets/images/docs/arch-overview/web-framework-diagram.png" alt="Flutter architecture for web" >
 
 ## What types of apps can I build?
+
 While you can do a lot on the web,
 Flutter's web support is most valuable in the
 following scenarios:

src/content/assets/images/docs/build-variant-menu.png

@@ -1112,8 +1112,7 @@ and falls back to CanvasKit otherwise.
 The draw.io source for the following image is in /diagrams/resources
 {% endcomment %}
 
-![Flutter web
-architecture](/assets/images/docs/arch-overview/web-framework-diagram.drawio.png){:width="100%"}
+![Flutter web architecture](/assets/images/docs/arch-overview/web-framework-diagram.png)
 
 Perhaps the most notable difference compared to other
 platforms on which Flutter runs is that there is no need

src/content/assets/images/docs/codelab/layout/businesscarddisplay1.png

@@ -161,11 +161,6 @@ or might partially or completely overlap.
 
 * [Staggered Animations][]
 
-{% comment %}
-  Save so I can remember how to add it back later.
-  <img src="/assets/images/docs/ic_new_releases_black_24px.svg" alt="this doc is new!"> NEW<br>
-{% endcomment -%}
-
 <a id="concepts"></a>
 
 ## Essential animation concepts and classes