chore(site): restructure, page titles & keywords

- add keywords,
- update page titles to include "workflow" and "open source"
- rename "Design" -> "BPMN 2.0"
- rename "Implement" -> "Documentation"
- re-order BPMN 2.0 Section, Tutorial and Reference come first
chore(site): fix accidental whitespace in title
feat(analytics): add Google Analytics event tracking for downloads