Move MACRO description to message_ix

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:
- [x] One on this repo, removing these sections and replacing with an intersphinx link to the correct section of the message_ix docs. —#51
- [ ] One on message_ix, adding the corresponding sections.

Review the MACRO description sections in this repository and the referenced message_ix documentation area. Remove the duplicated sections here and replace them with an intersphinx link, then coordinate the corresponding sections in message_ix; completion requires both linked documentation changes.

Written by the indexing model from the issue text.