Mention iiasa/message-trade in documentation

[khaeru]

Background:

• In 2019, @junukitashepard developed the code and data in iiasa/message-trade.
• This repo was added as a Git submodule of message_data, but not functionally integrated.
• Similar but improved features (under the name 'bilateral(ize)') are in development. These will land in (or at least be invoked via) message-ix-models.

iiasa/message_data#619 removes the submodule, consistent with the overall goal to slim down the repo and avoid potentially confusing duplication with message-ix-models by removing migrated, unused, superseded code.

However, it would still be good to retain pointers to the older code for reference, so colleagues can be aware of it, study it, and reuse any useful pieces. To achieve this, text and links should be added to the message-ix-models docs, perhaps as an extra section on a page describing the new/updated trade features.