Use jekyll as template engine as provided by GitHub

Avoid duplicated header and footer HTML source code by using jekyll.
This also fixes the title of spec.html and tools.html.
Fix URL to MessagePack

The MessagePack uses the TLD ‘org’ and not ‘com’.
Add scheme to all URIs

Some server deliver different content for http and https (e.g.
ietf.org).
Make the scheme explicit to avoid this problem.
Add 404.html page to server page not found errors