Make the possiblity to store stimuli files more "obvious"

[Remi-Gau]

Eventhough storing stimuli files is supported by BIDS, the information is "a bit hard to find".

For example, a search on the mkdocs does not bring up the section task events where the information is located (in the description of the stim_file column for events file).

Similarly, it is unclear at the moment if one can have stimuli in a BIDS dataset that are not referenced in any events file.

Would it make sense to take some of that content out of the table and possibly "combine" it with the sitmuli database section?

[Remi-Gau]

fixed by #697