Custom primitives example (#13795)

# Objective

- Add a new example showcasing how to add custom primitives and what you
can do with them.

## Solution

- Added a new example `custom_primitives` with a 2D heart shape
primitive highlighting
  - `Bounded2d` by implementing and visualising bounding shapes,
  - `Measured2d` by implementing it,
  - `Meshable` to show the shape on the screen
- The example also includes an `Extrusion<Heart>` implementing
  - `Measured3d`,
  - `Bounded3d` using the `BoundedExtrusion` trait and
  - meshing using the `Extrudable` trait.

## Additional information

Here are two images of the heart and its extrusion:

![image_2024-06-10_194631194](https://github.com/bevyengine/bevy/assets/62256001/53f1836c-df74-4ba6-85e9-fabdafa94c66)
![Screenshot 2024-06-10
194609](https://github.com/bevyengine/bevy/assets/62256001/b1630e71-6e94-4293-b7b5-da8d9cc98faf)

---------

Co-authored-by: Jakub Marcowski <37378746+Chubercik@users.noreply.github.com>

Cargo.toml

@@ -3024,6 +3024,17 @@ description = "Demonstrates all the primitives which can be sampled."
 category = "Math"
 wasm = true
 
+[[example]]
+name = "custom_primitives"
+path = "examples/math/custom_primitives.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.custom_primitives]
+name = "Custom Primitives"
+description = "Demonstrates how to add custom primitives and useful traits for them."
+category = "Math"
+wasm = true
+
 [[example]]
 name = "random_sampling"
 path = "examples/math/random_sampling.rs"

examples/README.md

@@ -329,6 +329,7 @@ Example | Description
 
 Example | Description
 --- | ---
+[Custom Primitives](../examples/math/custom_primitives.rs) | Demonstrates how to add custom primitives and useful traits for them.
 [Random Sampling](../examples/math/random_sampling.rs) | Demonstrates how to sample random points from mathematical primitives
 [Rendering Primitives](../examples/math/render_primitives.rs) | Shows off rendering for all math primitives as both Meshes and Gizmos
 [Sampling Primitives](../examples/math/sampling_primitives.rs) | Demonstrates all the primitives which can be sampled.