Move MACRO description to message_ix

[khaeru]

As suggested by @gidden on #27 by @volker-krey:

A lot of this text [describing MACRO per se] can be moved to the core message_ix docs, given that many of the operations are consistent across any implemented MESSAGE[-MACRO] model.

This will require two linked PRs:

• One on this repo, removing these sections and replacing with an intersphinx link to the correct section of the message_ix docs. — 2020 release of MESSAGEix-GLOBIOM documentation #51
• One on message_ix, adding the corresponding sections.