Add Documentation.yml GitHub action

[francescoalemanno]

Hi Tamas,

this way of building docs is much easier and cleaner than using travis,
the only caveat is:
JuliaDocs/Documenter.jl#1177
TL;DR:
the user has to trigger for the first time a build on the docs via a no-op commit to gh-pages branch
it still seems much prettier than adding keys to the repo manually

and if manually triggering github to rebuild the pages is too much to bear, the user can still add the traditional DOCUMENTER_KEY environment variable

all in all this way of deploying docs to me it seems to be much smoother to new users

let me know what you think

Best regards,

Francesco

[codecov]

Codecov Report

Merging #25 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   98.48%   98.48%           
=======================================
  Files           1        1           
  Lines          66       66           
=======================================
  Hits           65       65           
  Misses          1        1

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[anandijain]

Could you explain what you mean by no-op commit to gh-pages?
What should I do, so that the docs will start building?
Thanks!

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Edit: I think I got it, running deploydocs creates the branch, but you have to git checkout gh-pages and commit.