Sphinx autodocs deployment

[alphaville]

Main Changes :construction:

New GitHub Action which

• Installs sphinx and opengen
• checks out gh-pages and removes the folder api-dox
• commits and returns back to the original branch (this is probably an unnecessary commit)
• builds the documentation with sphinx-apidoc and make html
• adds the docs to gh-pages to be deployed

Moreover,

• added .nojekyll to gh-pages
• the API documentation is available here
• the folder test was moved outside the main source folder

To create the documentation locally, do the following:

1. outside the virtual environment install sphinx (ref) using

   pip install sphinx
   pip install sphinx-rtd-theme

2. Activate the virtual environment and install opengen (pip install .)
3. Go to sphinx-dox and run

   sphinx-apidoc -o ./source/ ../open-codegen/opengen ../open-codegen/opengen/test/
   make html

:children_crossing: this job will only run if the commit is on the master branch or if the commit message contains [docit]

Associated Issues :lady_beetle:

• Closes #306

Checklist :tractor:

• [x] Put a link in the main documentation page to the sphinx page