Add/improve docs tests

[shawnbot]

I noticed when going through each of the Primer CSS modules to update their test scripts in #334 that most are missing docs tests that ensure our documentation references all of the classes modified in the module's SCSS. Here's the list based on the presence of test-docs script in each module's package.json:

• primer-alerts
• primer-avatars
• primer-base
• primer-blankslate
• primer-box
• primer-breadcrumb
• primer-buttons
• primer-cards (soon to be deprecated)
• primer-core (meta)
• primer-css (meta)
• primer-dropdown
• primer-forms
• primer-labels
• primer-layout
• primer-markdown
• primer-marketing-support (support)
• primer-marketing-type
• primer-marketing-utilities
• primer-marketing
• primer-navigation
• primer-page-headers
• primer-page-sections
• primer-product (meta)
• primer-support (support)
• primer-table-object
• primer-tables
• primer-tooltips
• primer-truncate
• primer-utilities

[shawnbot]

@broccolini and I agreed that we should aim to add the docs test to all of the CSS modules and whitelist any classes that don't need to be explicitly documented.

[shawnbot]

/cc @emplums for future docs stuff