Add workflow that builds & deploys examples and docs

[david-pl]

Will certainly take some back and forth in order for this to work properly, but in the end this should

• clone the examples repo
• clone the docs repo (this one)
• install the latest QuantumOptics.jl release
• build the examples
• build the docs
• deploy the result on our website

[david-pl]

@amilsted @Krastanov @ChristophHotter okay so this is finally good to go. Some things to note:

• The workflow is triggered manually. Since when to update the documentation depends on the libraries as well as the examples and documentation repo, I thought it's best to leave it like this. You should be able to go to the Github Actions tab in the repo and select the workflow. There should be a button Run workflow. If you don't have access, please let me know.
• The following versions are used by the workflow:
  • The current master branch of the QuantumOptics.jl-examples repo.
  • The current master branch of the QuantumOptics.jl-examples repo.
  • The dependencies are installed according to the respective Project.toml files in the repository. Right now, there are no [compat] entries, which is sort of fine (we want to use the latest version of QO packages). However, we may want to think about adding compat boundaries for packages such as Documenter.jl. Anyway, we can set that directly in the Project.toml files, no need for changes in the workflow.
• While testing I already built everything and uploaded it to the homepage. It may be a good idea to check the docs & examples there to see if you notice anything weird.

Cheers,
David

[amilsted]

Heroic work - thanks @david-pl!