Improve documentation of the architecture and general principles

[robertoostenveld]

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 http://fieldtriptoolbox.org/development/architecture page to improve the documentation of these principles and features.