Dev documentation changes: Add slot to breadcrumbs

[daine]

📇 User story

As a developer, I want to add documentation about a slot in the breadcrumbs component so that other developers can understand how to use it effectively.

✅ Definition of Done / Outcomes

• [ ] Done when the website documentation is updated to have information about the default slot
• [ ] Done when storybook event and properties page has been updated to include the default slot

📜 Acceptance criteria

• [ ] The purpose of the slot in the gcds-breadcrumbs component should be clearly described.
• [ ] The documentation should explain what a slot is in the context of the breadcrumbs component.

📝 More info

Do we need content updates on the breadcrumbs documentation to describe the usage for the slot?

This issue was raised by a user through our contact us feedback form. Freshdesk ticket

[jprince-cds]

Please let the support team know once completed to circle back with Freshdesk #19253. Thanks!