[WIP] Build docs in parallel

[cpuhrsch]

This can reduce build time

[codecov[bot]]

Codecov Report

Merging #1159 (6eede29) into master (4b2dfb0) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1159   +/-   ##
=======================================
  Coverage   79.63%   79.63%           
=======================================
  Files          47       47           
  Lines        3201     3201           
=======================================
  Hits         2549     2549           
  Misses        652      652           

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