Add scripts, docs for migrating `message_data` code

[khaeru]

These describe/implement the process used for #88.

This PR can be updated as #88 (or #86, or another PR) is completed and merged.

How to review

Read the updated documentation.

PR checklist

• [x] Complete and merge #88; rebase this branch.
• [x] Add a reference to the info added to the description of #86 by @glatterf42.
• [x] Continuous integration checks all ✅
• [x] ~Add or expand tests;~ coverage checks both ✅
• [x] Add, expand, or update documentation.
• [x] Update doc/whatsnew.

[codecov[bot]]

Codecov Report

Merging #89 (e42fa68) into main (ecb3bb2) will decrease coverage by 0.1%. The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main     #89     +/-   ##
=======================================
- Coverage   68.4%   68.4%   -0.1%     
=======================================
  Files         63      63             
  Lines       4177    4176      -1     
=======================================
- Hits        2858    2857      -1     
  Misses      1319    1319             

Files Changed	Coverage Δ
message_ix_models/util/sdmx.py	100.0% <100.0%> (ø)

[khaeru]

For reference, here are the duplicates.txt and (manually edited) rebase-todo.txt produced by 2-migrate.sh while applying this process for #88:

• duplicates.txt
• rebase.txt

For example:

• Add water infrasructure [sp] as a commit message appears in duplicates.txt —the development history of the source repo/branch has resulted in duplicates of this commit.
• So, in editing rebase-todo.txt to rebase.txt, I removed one of the occurrences of this commit in the list of commands.

[khaeru]

@glatterf42 I've now completed the text and TODO list above, so this is ready to be reviewed and, if approved, merged.