Updates to ITP Onboarding

**Which module(s) and week(s) does this change affect?**
Module(s): Onboarding
Week(s): All

**What is the work that needs to be done?**

We are reorganising the first module of ITP to include JavaScript content. The majority of content will be pulled forward from Structuring & Testing Data and updated as necessary but some new material will be needed. Details of the proposed new structure are on a [Miro board](https://miro.com/app/board/uXjVGmM1MYA=/?share_link_id=890596212970), contact @cifarquhar for access if needed.

Sub-issues will be created with details of specific tasks and linked from here.

**Why is this work important to do?**

Onboarding is currently very light on content and doesn't give an accurate representation of what future weeks will look like. By adding more to the prep work trainees have a better idea of what they're signing up for before enrolling.

This also gives us an opportunity to spend time consolidating key topics such as Git usage, and opens up space in later modules to add more content there.

**How to contribute**

Contributions are welcome, but before picking something up please bear in mind:

- Work is being done on the `onboarding-updates` branch
- These changes are more involved than just adding a single piece of content or updating some code. They will fit together to form a coherent narrative through the module so communicating with other contributors will be critical. The [deploy preview](https://deploy-preview-1849--cyf-curriculum.netlify.app/) will show the current state of the module, but please ask questions too!
- Most files which have been moved from elsewhere have a red `CAUTION` box at the top highlighting potential issues
- New files have suggested key points in a bullet-point list. Feel free to adjust these as necessary, or to split/combine files if it makes more sense in context.