Docs links to `SNAPSHOTS` appear broken

[glatterf42]

message-ix-models/doc/api/model-snapshot.rst

Line 16 in c6285cf

…where :program:`0` is the ID of a snapshot; see :data:`.SNAPSHOTS`.

On our docs page instructing how to use the global baseline snapshot, the links to SNAPSHOTS seem to be broken, at least I can't click them.

[khaeru]

IIRC SNAPSHOTS was once separate, but then I noticed it contained the same data and served the same purpose as .util.pooch.SOURCE, so rolled them together.

[khaeru]

…we could, however, also add more verbose description of the individual snapshots and what they represent; this will be more important as more are published.