Track migration from message_data and message_single_country

[khaeru]

This PR exists solely to add this documentation page with a table for tracking work related to #275. See the "Files changed" tab for the ReST source of the page.

The PR may not ever be merged.

How to review

TBD

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew.

[khaeru]

(This comment is an example of how to contribute information about particular items.)

res_marg:

• I just diff'd the versions on m_data and m_s_c and found they are identical except for a print() statement (presumably added for debugging) in the latter. So the pick can be either.
• Opened Add .tools.res_marg #277 as an example PR.
• Updated the description table in this PR.

[khaeru]

During the 2025-02-27 MESSAGE team meeting, Oliver suggested that copy_parameter (perhaps better renamed copy_technology) and perform_vintage_check were two functions that could be first targets for migration.