Improve doctest handling

[yaugenst-flex]

Currently, doctests are not executed by runners, which can lead to deterioration of our docstring examples over time. The main issue is that for some examples, doctests might be annoying to set up properly, two cases in particular seem common (see #1797 and #1798 for example):

1. Setting up a full tidy3d simulation just to show an example for one simple API call
2. Docstrings failing because of logged warnings

So currently we solve 1. by either just setting up the simulation or skipping the test, and for 2. we basically just skip the test.
It would be good to find a way to do this without having to skip the tests altogether.