docs: use html for notebook download links

[wpbonelli]

• close notebooks: simplify .py/.ipynb download links #1860
• use suggested HTML for notebook prolog & download link
• import matplotlib in conf.py to prebuild font cache
• determine type of build from version string

New notebook header will look like

The .py link is to the source script in this repo (ref corresponding to the docs version), the notebook link displays the .ipynb source which can be downloaded with ctrl+s (previously the link was an instant download).

[codecov]

Codecov Report

Merging #1976 (f199c85) into develop (1197094) will decrease coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop   #1976     +/-   ##
=========================================
- Coverage     72.7%   72.7%   -0.1%     
=========================================
  Files          257     257             
  Lines        57810   57809      -1     
=========================================
- Hits         42081   42074      -7     
- Misses       15729   15735      +6     

see 3 files with indirect coverage changes