Add documentation rendering

[fsimonis]

This PR adds minimal documentation rendering using sphinx.

The porting and release guide are included but not modified.

The build.sh creates a venv with all dependencies, installs pyprecice and then builds the documentation as a singlehtml.
Note that the pyprecice package needs to be installed for autodoc to extract the docstrings etc.

As a cleanup, some utilities could be marked as private.

[IshaanDesai]

Thanks for this, much needed! It would make sense to add CI to re-render the docstrings documentation if it is updated via a PR or commit. Will this already be done via the submodules, or do we need to trigger it?

[fsimonis]

Instead of the website integration, a valid option could be to use read the docs as deployment method.

It allows specifying the latest Ubuntu lts as OS and pre-build commands, so one could install the latest preCICE.

This could be a robust deployment method.

[fsimonis]

I'll convert this to a draft until there is more certainty on how to proceed here precice/precice.github.io#630