refactor(links): add late linking support via docpad-plugin-links

Links in documents must now be specified via `ref:/some/link`.
This ensures that links are getting properly resolved via the
newly included docpad-plugin-links plugin.

BREAKING CHANGE:

The helpers @H(1..4), A and so forth have been removed.
In page links are now directly generated from the header names.

The helper @docUrl has been removed. Use `ref:/my/u/r/l` instead.

Related to CAM-1315
chore(real-life/examples): rename index page
fix(links): upgrade additional links

Related to CAM-1315
chore(user-guide/tasklist): remove too new documentation