Confirm we have all guidance that supports development (week 2 / 3)

[calvin-lau-sig7]

This issue is to gather code-related guidance needed in the GOV.UK header and Service header navigation components as they're identified as part of development work.

What

We have draft component guidance being reviewed, but will need to add more as dev work is finalised.

Why

It's important to gather these guidance needs as they come up in team conversations, however decisions and work might still need to be finalised before content work can start.

Who needs to work on this

Content designer, developer

Who needs to review this

Navigation squad

### Tasks
- [ ] Explain how the 'slots' work
- [ ] Warn services about risks with using slots
- [ ] Implementation of One Login (TBC with One Login team)
- [ ] Explain how service teams can 'unlink' current page so users better understand where they are – including both examples