Add doc for the ELEVATE project

[yiyi1991]

This PR adds the documentation for the ELEVATE project.

How to review

• Whether the elevate.rst contains all working packages related to IIASA,
• makes sense to internal modelers,
• and assigns the correct PL/PM.

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 74.7%. Comparing base (939ab94) to head (d678c37).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #427   +/-   ##
=====================================
  Coverage   74.7%   74.7%           
=====================================
  Files        270     270           
  Lines      22331   22331           
=====================================
  Hits       16688   16688           
  Misses      5643    5643           

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[khaeru]

IMO, good to go with these few changes and approval from the other reviewer.