Refine docs

Aligning with: https://github.com/thoughtbot/bourbon/pull/578

- Add “quick links” near top of README to docs, changelog and issues
- Tweak wording for installation
- Remove CLI section in favor of just pointing people to the wiki page
- Point Bourbon family links to GitHub repos instead of marketing sites
- Use new thoughtbot logo (SVG)
- Fixes #224
Why was this section removed? There is no other clear mention of the docs elsewhere...