Update developer documentation

[ia267]

• Added comments to all the source files using FORD format (see https://forddocs.readthedocs.io/en/stable/) - this was done using GitHub Copilot with Claude Sonnet 4.5
• The process was semi-automatic (the AI agent had missed some parts of the documentation, or some of it was incorrect). Most comments are not verified for correctness. When we are modifying some code we have to make sure that it is actually correct. For the code that I did look at, they were mostly correct (but for some parts of the code it had hallucinated and made up stuff)

For review I suggest looking at couple of source files that you're familiar with and check if the comments are OK (in terms of accuracy, verbosity).

Alternatively you could do the following:

• install ford using pip install ford
• go to the directory x3d2/docs and then run ford ford.md
• Then open the the x3d2/docs/doc/index.html in a browser

Close #247

[kaanolgu]

Initial check for poisson_fft and poisson_fft_cuda

[kaanolgu]

LGTM! (If anything else pops up, we can always address it in a follow-up PR)