Reproduce documentation from pubrules about specberus

[tripu]

Much related to issue #22; current action by @tripu is to implement the new approach to L10n and rule wording.

[tripu]

#160 set the basis for this: we have now in this repo all descriptions for all rules from the old pubrules checker (lib/rules.json; it's a bad filename). The mechanism to render those descriptions on the web UI is in place too.

This issue isn't solved though, because matching Specberus rules to the corresponding paragraphs from the old pubrules is, AFAIK, a manual task. We still need to write most selectors in lib/l10n-selectors.js.

[tripu]

Solved with #355 and a bunch of other related changes (some of them mentioned above).