Point to also Models and Apps

I keep having hard time finding them. ATM requires pretty much going through
BEP, google doc, github repo, url in description.   If those specifications are
parts of BIDS they are better be listed among SPECIFICATIONs drop down.

I also decided to point directly to thos bids-specification-pdf-releases/
since zenodo seems to offer only the .zip download where you then extact PDF.
From GitHub you can click "Download" right away

Author: yarikoptic

_data/navigation.yaml

@@ -14,8 +14,16 @@ menu:
         url: /benefits.html#mri-and-pet-converters
       - title: MEEG Converters
         url: /benefits.html#eeg-meg-ieeg-converters
-  - title: SPECIFICATION
-    url: /specification.html
+  - title: SPECIFICATIONS
+    sublinks:
+      - title: BIDS
+        url: https://bids-specification.readthedocs.io/en/stable/
+      - title: BIDS (PDF)
+        url: https://github.com/bids-standard/bids-specification-pdf-releases/blob/main/bids-spec.pdf
+      - title: BIDS Models
+        url: https://bids-standard.github.io/stats-models
+      - title: BIDS Apps
+        url: https://bids-standard.github.io/execution-spec/
   - title: GET STARTED
     url: /get_started.html
   - title: GET INVOLVED