Clean up doxygen comments in nestkernel directory

Final run-through and further cleanup of differen types of comments

Add a custom 404 page for Read the docs

Set copybutton to only copy input lines in docs

Update microcircuit docs regarding virtual process constraint and fix images

Fix formatting of developer guidelines - naming conventions

Fix typo on docs landing page

Remove redundant SLI tests

Enable patching of binary files within the online documentation

Add 'what's new' docs for 3.5

Port `issue-410.sli` to Pytest

Refactor SLI test for spike generator to Python

Co-authored-by: Nicolai Haug <39106781+nicolossus@users.noreply.github.com>

Fix formatting of `glif_psc` and `glif_cond` documentation

Port `test_sinusoidal_poisson_generator` from SLI-2-Py

Fix test

Merge branch 'port_test_sinusoidal_poisson_generator' of github.com:nicolossus/nest-simulator into nico_test_sin_poiss