Add example notebook on implementing custom elements to documentation

[jank324]

Description

Add a notebook that guides people along the implementation of new elements in Cheetah.

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Closes #396.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code and checked that formatting passes (required).
• I have have fixed all issues found by flake8 (required).
• I have ensured that all pytest tests pass (required).
• I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• I have checked that the documentation builds (required).

Note: We are using a maximum length of 88 characters per line.

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for implementing custom elements in the Cheetah accelerator physics library. The documentation consists of a Jupyter notebook that provides detailed guidance on extending Cheetah with user-defined elements.

• Adds a complete tutorial notebook showing how to subclass the Element base class
• Provides extensive code examples with TODO comments explaining each implementation step
• Includes guidelines for contributing new elements back to the main repository

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
docs/index.rst	Adds the new custom elements example to the documentation table of contents
docs/examples/custom_elements.ipynb	Complete tutorial notebook with detailed implementation guidance and code templates
CHANGELOG.md	Documents the addition of the new example notebook
.vscode/ltex.dictionary.en-GB.txt	Adds "subclassing" to the spell-check dictionary

[jank324]

@Hespe, when you review this, can you check the result in the built documentation. Can you also check if this matches the current version of Cheetah? I modelled this after when we did in #406.

I'm not sure if we should go ahead and merge this PR when it's ready, or wait for #476, because that will change some things.

Anyways, @frankmayet can have a look at this already.

[Hespe]

Notebooks on readthedocs don't seem to use the full screen width. Of course you can scroll horizontally, but this makes the docstrings anoying to read:

[Hespe]

I'm not sure if we should go ahead and merge this PR when it's ready, or wait for #476, because that will change some things.

I would be in favour of merging #476 first. Otherwise, I think its likely that we will not update this example soon enough. If we merge this PR first, #476 should in best case also update the example.

[jank324]

I'm not sure if we should go ahead and merge this PR when it's ready, or wait for #476, because that will change some things.

I would be in favour of merging #476 first. Otherwise, I think its likely that we will not update this example soon enough. If we merge this PR first, #476 should in best case also update the example.

Seconded!

[Copilot]

Pull request overview

Copilot reviewed 24 out of 24 changed files in this pull request and generated 3 comments.

[jank324]

@Hespe, small reminder.

[Hespe]

Looks mostly good to me. However, the example did not talk at all about serialization and converters. I think we should at least mention that.

[jank324]

Looks mostly good to me. However, the example did not talk at all about serialization and converters. I think we should at least mention that.

Like the latest revision?

[Hespe]

Looks mostly good to me. However, the example did not talk at all about serialization and converters. I think we should at least mention that.

Like the latest revision?

Yes, but the formatting is not consistent with the rest of the document. They are the only bold headers throughout.

[jank324]

Looks mostly good to me. However, the example did not talk at all about serialization and converters. I think we should at least mention that.

Like the latest revision?

Yes, but the formatting is not consistent with the rest of the document. They are the only bold headers throughout.

Now it matches?