Implement GDS colors

• Implements GitLab Design System's colors in the docs site (neutral and blue palettes)
• Adjust sections with gray elements to GDS colors and proper contrast levels (for https://gitlab.com/gitlab-com/gitlab-docs/issues/298):
  • Sidebars (ToC and nav)
  • Feedback
  • Versions dropdown menu
• Adjust major elements colors
  • Body font and background
  • h1 border-bottom
  • h* border-bottom
  • Dividers and borders
  • Table headings and borders
  • Bootstraps default button

Note that I'm also using the lighter gray possible for backgrounds, to make the overall look'n'feel lighter. Please let me know what y'all think about it. :)

ToC, nav, and h1 border-bottom, table's header

BEFORE	AFTER

Blue links

BEFORE NORMAL	BEFORE HOVER	AFTER NORMAL	AFTER HOVER

Feedback section

BEFORE	AFTER

Versions dropdown

BEFORE	AFTER

Default button

BEFORE	AFTER

Review aap: http://implement-gds-colors.178.62.207.141.xip.io/ee/

cc/ @gl-docsteam FYI