Closes https://github.com/twbs/bootstrap/issues/17493

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color

move compilting to build-tools, rewrite some grunt stuff, add local docs with jekyll section based on readme's more detailed walkthrough