Doc/module style doc blocks for examples (bevyengine#4438)

# Objective

Provide a starting point for bevyengine#3951, or a partial solution. 
Providing a few comment blocks to discuss, and hopefully find better one in the process. 

## Solution

Since I am pretty new to pretty much anything in this context, I figured I'd just start with a draft for some file level doc blocks. For some of them I found more relevant details (or at least things I considered interessting), for some others there is less. 

## Changelog

- Moved some existing comments from main() functions in the 2d examples to the file header level
- Wrote some more comment blocks for most other 2d examples

TODO: 
- [x] 2d/sprite_sheet, wasnt able to come up with something good yet 
- [x] all other example groups...


Also: Please let me know if the commit style is okay, or to verbose. I could certainly squash these things, or add more details if needed. 
I also hope its okay to raise this PR this early, with just a few files changed. Took me long enough and I dont wanted to let it go to waste because I lost motivation to do the whole thing. Additionally I am somewhat uncertain over the style and contents of the commets. So let me know what you thing please.

Author: themasch

examples/2d/mesh2d.rs

@@ -1,3 +1,5 @@
+//! Shows how to render a polygonal [`Mesh`], generated from a [`Quad`] primitive, in a 2D scene.
+
 use bevy::{prelude::*, sprite::MaterialMesh2dBundle};
 
 fn main() {

examples/2d/mesh2d_manual.rs

@@ -1,3 +1,8 @@
+//! This example shows how to manually render 2d items using "mid level render apis" with a custom
+//! pipeline for 2d meshes.
+//! It doesn't use the [`Material2d`] abstraction, but changes the vertex buffer to include vertex color.
+//! Check out the "mesh2d" example for simpler / higher level 2d meshes.
+
 use bevy::{
     core_pipeline::Transparent2d,
     prelude::*,
@@ -23,9 +28,6 @@ use bevy::{
     utils::FloatOrd,
 };
 
-/// This example shows how to manually render 2d items using "mid level render apis" with a custom pipeline for 2d meshes
-/// It doesn't use the [`Material2d`] abstraction, but changes the vertex buffer to include vertex color
-/// Check out the "mesh2d" example for simpler / higher level 2d meshes
 fn main() {
     App::new()
         .add_plugins(DefaultPlugins)

examples/2d/move_sprite.rs

@@ -1,3 +1,5 @@
+//! Renders a 2D scene containing a single, moving sprite.
+
 use bevy::prelude::*;
 
 fn main() {
@@ -25,6 +27,8 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
         .insert(Direction::Up);
 }
 
+/// The sprite is animated by changing its translation depending on the time that has passed since
+/// the last frame.
 fn sprite_movement(time: Res<Time>, mut sprite_position: Query<(&mut Direction, &mut Transform)>) {
     for (mut logo, mut transform) in sprite_position.iter_mut() {
         match *logo {

examples/2d/rotation.rs

@@ -1,3 +1,5 @@
+//! Demonstrates rotating entities in 2D using quaternions.
+
 use bevy::{
     core::FixedTimestep,
     math::{const_vec2, Vec3Swizzles},

examples/2d/shapes.rs

@@ -1,3 +1,5 @@
+//! Shows how to render simple primitive shapes with a single color.
+
 use bevy::{prelude::*, sprite::MaterialMesh2dBundle};
 
 fn main() {

examples/2d/sprite.rs

@@ -1,3 +1,5 @@
+//! Displays a single [`Sprite`], created from an image.
+
 use bevy::prelude::*;
 
 fn main() {

examples/2d/sprite_flipping.rs

@@ -1,3 +1,5 @@
+//! Displays a single [`Sprite`], created from an image, but flipped on one axis.
+
 use bevy::prelude::*;
 
 fn main() {

examples/2d/sprite_sheet.rs

@@ -1,3 +1,6 @@
+//! Renders an animated sprite by loading all animation frames from a single image (a sprite sheet)
+//! into a texture atlas, and changing the displayed image periodically.
+
 use bevy::prelude::*;
 
 fn main() {

examples/2d/text2d.rs

@@ -1,3 +1,10 @@
+//! Shows text rendering with moving, rotating and scaling text.
+//!
+//! Note that this uses [`Text2dBundle`] to display text alongside your other entities in a 2D scene.
+//!
+//! For an example on how to render text as part of a user interface, independent from the world
+//! viewport, you may want to look at `2d/contributors.rs` or `ui/text.rs`.
+
 use bevy::{prelude::*, text::Text2dBounds};
 
 fn main() {

examples/2d/texture_atlas.rs

@@ -1,7 +1,8 @@
+//! In this example we generate a new texture atlas (sprite sheet) from a folder containing
+//! individual sprites.
+
 use bevy::{asset::LoadState, prelude::*};
 
-/// In this example we generate a new texture atlas (sprite sheet) from a folder containing
-/// individual sprites
 fn main() {
     App::new()
         .init_resource::<RpgSpriteHandles>()