Update docs for 2025/SSP 3.x/ScenarioMIP-CMIP7

[khaeru]

This PR and branch exist to collect revisions to the ‘conceptual’ docs as part of #424.

How to review

View the ReadTheDocs preview build from this branch, particularly the page “MESSAGEix-GLOBIOM global model” and subpages, but also other pages that are changed.

More TBA

PR checklist

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.4%. Comparing base (67854b9) to head (57aa438).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #425     +/-   ##
=======================================
- Coverage   74.7%   70.4%   -4.3%     
=======================================
  Files        272     272             
  Lines      22463   22463             
=======================================
- Hits       16784   15827    -957     
- Misses      5679    6636    +957     

see 11 files with indirect coverage changes

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