Documentation - add programming model page

[Markus-Goetz]

Feature functionality
Add a page describing HeAT's programming model.

[ClaudiaComito]

This would be great to have. Let me know how I can support you @Markus-Goetz

[Markus-Goetz]

Essentially text writing - it would be an explanation of the (relaxed) bulk synchronous processing (BSP) model including the graphic from the paper. Mainly boils down to saying that there are fixed synchronization points and inbetween there is arbitrary and concurrent computation and communication

[Markus-Goetz]

Perhaps it can be enriched by extending or having an extra page that describes on how to think about implementing a distributed algorithm, i.e. if not split just compute locally, otherwise communicate inbetween using MPI

[github-actions]

Branch docs/613-Documentation_-_add_programming_model_page created!

[mrfh92]

Clearly a good idea. Could be interesting for a student project (e.g. bachelor/master thesis) where some general explanatory/introductory text that is required within the thesis anyway can be used here too.

(Reviewed within #1109)