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 74.8%. Comparing base (f580412) to head (7bc59e2).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #427     +/-   ##
=======================================
- Coverage   75.5%   74.8%   -0.7%     
=======================================
  Files        270     270             
  Lines      22093   22093             
=======================================
- Hits       16687   16539    -148     
- Misses      5406    5554    +148     

see 7 files with indirect coverage changes

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[khaeru]

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