fix(docs): update styleguidist internal components to pass WCAG 2.0 compliance

[austingreendev]

This is a small sub-set of the changes originally proposed in #66

Description

This PR helps increase the accessibility of the internal styleguidist components to pass WCAG 2.0 compliance (originally discussed in #66)

Detail

This PR specifically:

• Creates a custom ArgumentsRenderer to use Garden colors
• Adds a top-level role to the LogoRenderer component along with screen-reader legible text
• Adds landmark region to TableofContentsRenderer
• Creates a custom TableRenderer to use our Garden table for prop-types display
• Override hljs theme with Github + accessible Garden for accessible code example syntax coloring

Before

Now

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Pull Request Test Coverage Report for Build 379

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.144%

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Totals
Change from base Build 375:	0.0%
Covered Lines:	1683
Relevant Lines:	1697

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💛 - Coveralls