Add content bootstraping script (script/bootstrap-codeforces)

This script bootstraps a single codeforces round,
including round index page and pages for each problem in the
match.
Add navigation for top-level items

All the rounds for each top-level items (e.g. SRM/CF) are
automatically scanned and listed at the index page.
Additionaly, each single item now has 'article' kind attribute.