docs: FIX quiver-scales in metrics.

-- Linked doc-pages with DISQUS to receive comments.
-- rtd: DIX docs, Still no plot-directive (Metrics).
-- metrics: Interactive pick-event & finalize arrow-diffs.
-- Reorder metrics in "contribute" section.
-- conf: Update MathJAX link.
- build: Simplify py-ver detection on `setup.py` withoutut StrictVersion
import.
[ci skip]