Use Turing Actions for Documenter site and Navbar

[shravanngoswamii]

Until now docs were using old navbar injection script that breaks at many places. I have replaced it with Turing Actions and also updated docs workflow to use Turing Actions which supports adding navbar direectory.

I have also updated navbar design to support all Documenter themes and lets keep JuliaGP navbar in this org instead of Turing so I have added new it in .github repo: JuliaGaussianProcesses/.github#4

Closes #576 & #565

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.46%. Comparing base (fcae30b) to head (23a73ab).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #593   +/-   ##
=======================================
  Coverage   94.46%   94.46%           
=======================================
  Files          52       52           
  Lines        1463     1463           
=======================================
  Hits         1382     1382           
  Misses         81       81           

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[github-actions]

KernelFunctions.jl documentation for PR #593 is available at:
https://JuliaGaussianProcesses.github.io/KernelFunctions.jl/previews/PR593/