Add guidance for fonts to be used within internal services

[alexnewmannn]

Related documentation

There's often repeated discussion about the use of GDS transport, the guidance isn't necessarily 100% clear on what to do for agent facing systems that reside on the gov.uk subdomain. Most internal services follow the advice not to use GDS transport, however, a consistent a font stack should be recommended for services to use. gov.uk recommend "an alternative font like arial" and this is the consensus within DWP, however, some clear guidance on what to use and an explanation of why will be useful.

Suggestion

A section on typography/fonts with an explanation on when you can and can't use GDS transport, and what to do if you can't use it. Ideally the font stack (and any other appropriate css that may be required) will be shown and teams will adopt.

Evidence (where applicable)

https://designnotes.blog.gov.uk/2015/03/11/can-i-use-the-gov-uk-fonts/ - This is the most clearcut guidance, at first when I saw "service.gov.uk" I thought "service" was a placeholder, not an actual part of the domain.
https://design-system.service.gov.uk/styles/typography/#when-not-to-use-the-gds-transport-font

[martinwake]

There is also this, in "Designing services for government users": https://www.gov.uk/service-manual/design/services-for-government-users#logos-and-fonts

You also will not be able to use the New Transport font. Use the Helvetica or Arial CSS font stacks instead.