Improve Mesh documentation (#9061)

# Objective

This PR continues bevyengine/bevy#8885

It aims to improve the `Mesh` documentation in the following ways:
- Put everything at the "top level" instead of the "impl".
- Explain better what is a Mesh, how it can be created, and that it can
be edited.
- Explain it can be used with a `Material`, and mention
`StandardMaterial`, `PbrBundle`, `ColorMaterial`, and
`ColorMesh2dBundle` since those cover most cases
- Mention the glTF/Bevy vocabulary discrepancy for "Mesh"
- Add an image for the example
- Various nitpicky modifications

## Note

- The image I added is 90.3ko which I think is small enough?
- Since rustdoc doesn't allow cross-reference not in dependencies of a
subcrate [yet](rust-lang/rust#74481), I have a
lot of backtick references that are not links :(
- Since rustdoc doesn't allow linking to code in the crate (?) I put
link to github directly.
- Since rustdoc doesn't allow embed images in doc
[yet](rust-lang/rust#32104), maybe
[soon](rust-lang/rfcs#3397), I had to put only a
link to the image. I don't think it's worth adding
[embed_doc_image](https://docs.rs/embed-doc-image/latest/embed_doc_image/)
as a dependency for this.

src/lib.rs

@@ -60,7 +60,8 @@ impl Plugin for GltfPlugin {
     }
 }
 
-/// Representation of a loaded glTF file.
+/// Representation of a loaded glTF file
+/// (file loaded via the `AssetServer` with the extension `.glb` or `.gltf`).
 #[derive(Debug, TypeUuid, TypePath)]
 #[uuid = "5c7d5f8a-f7b0-4e45-a09e-406c0372fea2"]
 pub struct Gltf {