Improve documentation of the architecture and general principles

On the train back from BIOMAG2022 in Birmingham, @schoffelen and I identified the following key features that we build on

- cfg (ft_checkconfig, user-readable)
- data structures (ft_checkdata, dimord, independent of file format, parsimonious)
- difference between high-level and (private) low-level functions, synchronize-private
- modular design of algorithms (specest, preproc, inverse, ...)
- pre/postamble
- inputfile/outputfile
- consistent user feedback (ft_notification)
- FT functions callling FT functions (rollback provenance)
- GUI layout ft_uilayout
- consistency in the reference documentation (i.e. help)

I have started on a page to improve the documentation of these principles and features.