Update guidance on using serviceName instead of productName

[matteason]

While answering this question on Slack I noticed the macro guidance for productName in the header hasn't been updated to reflect the fact that the service name should now go in the service navigation rather than in the header.

Questions:

• Is the absolute path in the link ok or should it have the domain name?
• Is the lowercase capitalisation of 'service navigation' in the link text ok or should it be 'Service navigation'?
• As mentioned on Slack, the distinction between product name and service name is quite buried, and you might not find it if you're not using the Nunjucks templates. Is it worth considering some guidance on this in the main page content? I guess this would need an issue in the Design System rather than Frontend

[owenatgov]

Sorry this has languished a bit @matteason. The team are in agreement this is worthwhile.

This needs a rebase first of all because we've changed that description to remove the reference to serviceName. However it's still worth steering folks away from using product name if they're one of the GOV.UK 'ingredient brands'. I'm happy to rebase this if you don't have time or don't feel confident.

We also don't use relative paths I think anywhere in the macro docs so we'd need to test that against the website to be sure it's not gonna cause problems.

I also personally wonder if we should be even more blunt here. Eg: 'the product name can only be used by GOV.UK/GDS products and is not for use by most services'. That's off the dome so I'd like the team to chime in on that before putting it forward as a suggestion.