Add template pages for user documentation

Description

Currently, we only provide API documentation for the C++ classes/methods in Doxygen. This is useful for people developing extensions for the solver but is rather unuseful for users. The temporal solution right now is to write everything in the README. This is not optimal if we want to have decent user documentation.

Proposal

Choose and add a static site generator (SSG) that turns hand-written documentation into something useful for users (e.g. webpage, pdf). After searching a while I came up with Docusaurus which seems to be a good compromise in complexity, flexibility and design. Since the documentation will be written GitHub-flavored Markdown syntax which means that it will be suitable for simple GitLab Markdown renders and will be esaly included in other projects documentation (e.g. spatial-model-editor written with sphinix).

How to test the implementation?

• Make a new job that builds the documentation and exports it as an artefact.
• Build and deploy the docs for an external service (e.g. GitLab/GitHub Pages, Netlify)

Related issues

None