Add docs page for model names

[khaeru]

Including at a minimum:

MESSAGEix-GLOBIOM 2.0

• R12 nodes.
• "B" list of year codes.
• Using message_ix_models.model.material.

…and then describing how various postfixes like -R17-T-DACCS both have been and should be used. Some heuristics there include:

• Omit these postfixes if the default settings have been used.
• Use features ("DACCS"), rather than project names ("NGFS").
• Use a certain order TBD.

This can be the central point of a list to be later extended with older and newer names.

Discussed at today's MESSAGE team meeting.