Reduce number of doctest runs

[lgoettgens]

Currently we run the doctests in CI in the following cases:

1. In each CI / test run of the julia version-OS-matrix once via

   GAP.jl/test/runtests.jl

   Line 26 in a8f197a

   include("doctest.jl")

2. In the CI / Documentation job twice. One time explicitly from the workflow yml configuration file, and another time by passing doctest = true to makedocs.

I don't really care about the makedocs case. But I really would like to remove the step from the documentation building workflow as a preliminary step towards working on #964.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.65%. Comparing base (be6e4e2) to head (e5ef53e).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1129      +/-   ##
==========================================
+ Coverage   74.66%   81.65%   +6.98%     
==========================================
  Files          55       50       -5     
  Lines        4713     3821     -892     
==========================================
- Hits         3519     3120     -399     
+ Misses       1194      701     -493     

see 8 files with indirect coverage changes