Add in-spec hyperlinks to make it more navigable

Slight improvement of hypelinks in the spec rules, since we have anchors, let's use them.

This is not urgent at all and probably should be done after @viktorklang's update the the rules in that PR: https://github.com/reactive-streams/reactive-streams/pull/117

Changes:
* Adds in–spec links between "see x.xx" elements
* Links to footnotes
* 4.1 (processor rules) was referencing to `[1]` but there's no `1` footnote for this section...? Removed the reference.