add javascript to insert download buttons

[snoyer]

in order to have big "get F3D button" for the current OS above the release table:

Include a script in the header, the script is loaded on every page but will perform different actions depending on which page it's in.
The idea is that the javascript only enhances the existing html, while being totally optional. We don't want to start adding <script> tags in the markdown documents.

In the case of the install page, the script retrieves the download links from the table in the page (html generated from the markdown file) and adds buttons for the ones where the "Platform" column matches the guessed client OS

edit: temporarily deployed at https://snoyer.github.io/f3d/doc/user/INSTALLATION.html
should look something like:

[mwestphal]

add a screenshot in description ? :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b02ce73) 96.51% compared to head (2f8f792) 96.51%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1266   +/-   ##
=======================================
  Coverage   96.51%   96.51%           
=======================================
  Files         131      131           
  Lines        8202     8208    +6     
=======================================
+ Hits         7916     7922    +6     
  Misses        286      286           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.