Type spacing updates

[Scotchester]

Updates for our latest type spacing guidelines.

Additions

• New copy and images for type spacing section.

  Removals

• Images that are no longer needed

  Changes

• Change all instances of "header" to "heading"
• Rename Web "subheading" to "lead paragraph"

  Testing

• http://scotchester.github.io/design-manual/identity/typography.html#spacing

  Review

• @nataliafitzgerald
• @jimmynotjim

  Checklist

• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows the standards laid out in the front end playbook
• [x] Passes all existing automated tests
• [x] New functions include new tests
• [x] New functions are documented (with a description, list of inputs, and expected output)
• [x] Placeholder code is flagged
• [x] Visually tested in supported browsers and devices
• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[jimmynotjim]

One small comment, otherwise :+1:

[jimmynotjim]

Whoops, running this locally and "Display Heading" and "Lead Paragraph" aren't rendering

[Scotchester]

Have you installed the latest cf-core?

[jimmynotjim]

Ha, you'd think I'd know that seeing how I made that point last week. Must be Monday. Looks good :+1: from me again.

[jimmynotjim]

@cfarm @KimberlyMunoz @ascott1 anyone else want to sign off on this?

[Scotchester]

@nataliafitzgerald gave me a verbal thumbs-up to this last week.