[css-box-3] Add mention of css-logical into introduction section

[himorin]

coming from i18n review (w3c/i18n-request#135 (comment)) for https://www.w3.org/TR/css-box-3/, but this is a suggestion and not a part of change request.

In section 3 and 4, logical properties, which are defined in css-logical and will be mapped to properties defined in this spec, are mentioned in their introduction part, like following text. But nothing is mentioned in section 1. introduction.
This specification defines the physical margin longhands; CSS Logical Properties 1 §4.2 Flow-relative Margins: the margin-block-start, margin-block-end, margin-inline-start, margin-inline-end properties and margin-block and margin-inline shorthands additionally defines flow-relative margin longhands.

Adding mentioning of such possible mappings into introduction could make people easy to tell what might be affected to properties defined in this spec.

[fantasai]

@himorin OK, I've added a reference from the introduction (and shortened the references from other sections). Let me know if this is acceptable!