Improve documentation

[kordejong]

Keep this issue open and use the associated branch to work on documentation. Once in a while merge the content into the master branch, using a PR, but keep the branch open and use new PRs over time.

Content to add:

• Add initial documentation for creating and executing simulation models
• Man-pages for the framework operations. For this we need to have a public API defined. See also Harmonize public C++ and Python APIs #639
• Policies
• Howto for adding a new local operation to LUE itself
• Howto for creating custom local operation, using LUE as 3rd party library
• List potential use-case for LUE, besides using it as is for model development
• How to use lue_qa for performance and scalability analyses
• ...