enable trigger-docs-generation for master branch as well

[st--]

(Hopefully) fixes #1404

[codecov]

Codecov Report

Merging #1580 into develop will increase coverage by 51.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           develop    #1580       +/-   ##
============================================
+ Coverage    44.02%   95.14%   +51.11%     
============================================
  Files           86       86               
  Lines         3848     3848               
============================================
+ Hits          1694     3661     +1967     
+ Misses        2154      187     -1967     

Impacted Files	Coverage Δ
gpflow/inducing_variables/inducing_variables.py	100.00% <0.00%> (+4.16%)	⬆️
...ducing_variables/multioutput/inducing_variables.py	92.85% <0.00%> (+7.14%)	⬆️
gpflow/models/sgpmc.py	94.59% <0.00%> (+16.21%)	⬆️
gpflow/base.py	91.83% <0.00%> (+18.36%)	⬆️
gpflow/quadrature/gauss_hermite.py	100.00% <0.00%> (+20.93%)	⬆️
gpflow/models/gpmc.py	94.87% <0.00%> (+23.07%)	⬆️
gpflow/models/util.py	76.92% <0.00%> (+23.07%)	⬆️
gpflow/models/training_mixins.py	88.23% <0.00%> (+23.52%)	⬆️
gpflow/covariances/multioutput/kufs.py	90.32% <0.00%> (+25.80%)	⬆️
gpflow/kernels/linears.py	100.00% <0.00%> (+25.92%)	⬆️
... and 52 more

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