Add doc for EFC projects

[yiyi1991]

This PR adds the documentation for the EFC projects.

How to review

• Whether the efc.rst contains all research activities related to MESSAGEix model development,
• 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 (3d28157).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #426     +/-   ##
=======================================
- 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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[khaeru]

@yiyi1991 thanks for this—especially for back-filling details about previous iterations/phases of the same project.

I made a few suggestions, see below. Once these are made and CI passes again, we can merge.

[khaeru]

Once these are made and CI passes again, we can merge.

I suppose we should also wait for ✅ from the 2 other requested reviewers, i.e. all 3 are needed.

[junukitashepard]

Made a small suggestion but otherwise looks good to me. Thanks!