Update documentation for ItemList and content in Timeline component

[MagdalenaLarge]

As Software Developer
I want Timeline component to have updated documentation for two parameters - ItemList and content
So that user knows dependencies within this component

Description
For the timeline component, we need to update the documentation for two parameters - ItemList and content. The description should be updated to clarify their dependency on each other. Specifically, itemList should only be set when content is not set, and vice versa.

Currently documentation is not stating this dependency clearly in Nunjucks micro options

Acceptance Criteria

1. Documentation for two parameters ItemList and content is updated
2. User can clearly identify dependencies for both parameters

Visualisation/Input from UCD

Visual representation where the issue is currently in Design System.

Linked User stories/epics

Technical details (optional)
MVP (optional)