Add content container token

Bookmarking for investigation alongside various other new tokens to add:

> Out of scope for this PR, but this is probably a magic number value that needs to become a token since we're using it to space things in relation to the Header constraints. I end up using it in apps that use the BCDS for layout purposes.

_Originally posted by @ty2k in https://github.com/bcgov/design-system/pull/678#discussion_r3275919931_