Improve server replace guide

[lornajane]

We have a guide for replacing the server details, but it is outdated.

We'd love to get some help with:

• [ ] updating the copy to explain why each section is needed/useful, and to use inclusive and friendly language
• [ ] check all the steps actually work
• [ ] Remove the "Step 1" bits from the titles, it makes the documentation harder to maintain if we have to re-number things when adding/removing steps.

Feel free to add any other changes that seem useful in this guide.