[ENH] Add links to example datasets for each modality

[Remi-Gau]

Fixes #842

Add links to example from the BIDS example repo in the following section:

• MEG
• iEEG
• genetics
• PET
• MRI
  • anat
  • func
  • dwi
  • fmap
  • asl
  • qmri
• behavior ?
• physio

[Remi-Gau]

While working on this I am tempted to split the table in the README of BIDS-example to match better with the BEP:
https://github.com/bids-standard/bids-examples#dataset-index

• get the PET example out of the multomodal table
• get the qMRI and the ASL examples out of the MRI dataset and in their own respective tables

I would be easier to then have a single link to those sub-tables in the spec, than a whole list of links in the spec. Also this way we can add more examples in BIDS example without having to do the same in the spec.

[sappelhoff]

I am tempted to split the table in the README of BIDS-example to match better with the BEP

Sounds good to me - perhaps also a nice opportunity to resolve bids-standard/bids-examples#224 (or close it in favor of your alternative solution)

[Remi-Gau]

I am tempted to split the table in the README of BIDS-example to match better with the BEP

Sounds good to me - perhaps also a nice opportunity to resolve bids-standard/bids-examples#224 (or close it in favor of your alternative solution)

Let me start tackling this one too.

[sappelhoff]

LGTM! Thanks Remi.

[tsalo]

Looks good to me. Thanks!