Use JuliaGPsDocs for the examples

[theogf]

Same as https://github.com/JuliaGaussianProcesses/AbstractGPs.jl/pull/312

Also need to update examples/README.md

[codecov[bot]]

Codecov Report

Merging #443 (708c0e9) into master (cb6357a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #443   +/-   ##
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  Coverage   93.14%   93.14%           
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  Files          52       52           
  Lines        1254     1254           
=======================================
  Hits         1168     1168           
  Misses         86       86           

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[st--]

@theogf I can't leave it as a suggestion but you can now replace the doctestfilters argument with

doctestfilters=JuliaGPsDocs.DOCTEST_FILTERS