JBIDE-16364 - Improve site performances

Apply suggestions reported by Chrome Page Speed:
- avoid redirects (updated location for some assets)
- use minified versions of css, js, html when possible
- compressed hero-unit image on home page (using jpg instead of png)
- avoid HTTP redirects
- specify images size
- use minified css and js
- upgrade to awestruct-0.5.5 and asciidoctor-1.5.1