Add GitHub workflows for CI and Book

[nickelc]

Adds two workflows based on the Travis and Azure setup.

I haven't looked into the Codecov stuff whether an additional secret is required for the CI workflow
or how it could be possible to use the Codecov Action.

The Book workflows requires a ACTION_DEPLOY_KEY secret for deployment to GitHub Pages.
See peaceiris/actions-gh-pages#1-add-ssh-deploy-key for the configuration.
The secrets.GITHUB_TOKEN token wouldn't trigger a page rebuild.

I don't know what's wrong with the macOS nightly run.

related to: #453

[codecov-io]

Codecov Report

Merging #487 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
- Coverage    85.7%   85.69%   -0.01%     
==========================================
  Files         111      111              
  Lines       15938    15938              
==========================================
- Hits        13659    13658       -1     
- Misses       2279     2280       +1

Impacted Files	Coverage Δ
juniper/src/http/playground.rs	0% <0%> (-25%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1b71ea6...f58a8b2. Read the comment docs.

[LegNeato]

Thanks! I set up ACTIONS_DEPLOY_KEY. Let's iterate on this on master.