Use GH actions to deploy documentation to GH pages

[GPMueller]

Since spirit now uses GH actions as CI, it could also be used to generate the documentation.
The most promising approach I have found so far would be to use mkdocs, the mkdocs-material theme and mike to generate version-specific sites and to deploy to GH pages. However, the Python package docs would need to be created in markdown and versioning is currently only available to "insiders".

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Steps would be:

• remove all unnecessary README files
• remove all .rst files and replace with .md files if necessary, potentially move contents to files in docs/ and core/docs
• write a script to generate python package docs and remove conf.py (run sphinx-apidoc and convert the resulting .rst files to .md)
• create a workaround for mkdocs' ridiculous limitations, e.g. by creating a link from the folder above spirit to spirit/docs/mkdocs.yml in which docs_dir: spirit

Open questions:

• deploy docs only if core builds were successful?

Related issues:

• Docs: improve Python docs formatting and parameter descriptions #533

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The folder structure in https://github.com/spirit-code/spirit-code.github.io could be something like

├── index.html
├── app_mobile.html
├── app_desktop.html
├── css
├── docs
│   ├── docs.spirit-code.github.io/latest
│   ├── docs.spirit-code.github.io/2.1.0
│   └── docs.spirit-code.github.io/3.0.0
├── img
├── js
└── mov

[GPMueller]

Alternative approach to creating links:
use CMake to

• install the README.md and LICENSE.txt to build/install/docs
• install the docs folder to build/install/docs
• install the core/docs folder to build/install/docs/core/docs (or build/install/docs/core?)
• install the docs/mkdocs.yml to build/install removing the need to specify docs_dir (default is docs_dir: docs)

Depending on the structure of the install-folder, links between markdown-files may be incorrect, either on GitHub or in the documentation.