Reduce the prominence of the newsletter sign up

[robredpath]

The footer page uses examples with the newsletter signup prolifically and so it's not surprising that people using the design system are including it where it's not needed.

Can we make it clearer in the examples and docs that the newsletter layout is there if we need it, but isn't to be included all over the place?

[jarofgreen]

And also make clear what "if we need it" means?

Cos I just assumed I should take it cos it's in the design theme, but apparently that was wrong