[FIX] Add `rawdata` to glossary

[kabilar]

Changes

• [x] Add the rawdata description to the glossary by combining text from other parts of the BIDS specification.

[effigies]

Rawdata is not a BIDS term. It's a name used in an example, but BIDS does not specify the name of the directory containing a BIDS dataset.

[Remi-Gau]

@effigies do you think we should probably clarify this? See https://github.com/bids-standard/bids-specification/issues/1519