Improve transport documentation

[khaeru]

This PR extends and improves the documentation of MESSAGEix-Transport, mainly to serve as extended/exhaustive supplementary information for the in-preparation manuscript about the model.

• Transcribe draft text (from the "SSP Transport writeup.docx" SharePoint document) into a new page.
• Copyedit the text.
• Reorganize existing transport docs for clarity.

How to review

• Look at the transport pages in the ReadTheDocs preview build of the documentation:
  • transport/index
  • transport/ssp —new page based on "SSP Transport writeup.docx".

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 73.3%. Comparing base (ac48376) to head (412bb9c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #473   +/-   ##
=====================================
  Coverage   73.3%   73.3%           
=====================================
  Files        301     301           
  Lines      24427   24427           
=====================================
  Hits       17912   17912           
  Misses      6515    6515           

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