Add doc/material/index to doc/index

[khaeru]

This was missed in #189, and as a result one can't see the new module in the "Model variants" section of the left navbar or the ToC on the index page.

How to review

Look at the RTD docs build for this PR and confirm that .model.material appears in both places: https://iiasa-energy-program-message-ix--196.com.readthedocs.build/projects/models/en/196/

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update doc/whatsnew. Silent clean-up of PR that's already mentioned.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.1%. Comparing base (e76bbd3) to head (8cb79c6).
Report is 1664 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #196     +/-   ##
=======================================
- Coverage   52.2%   52.1%   -0.1%     
=======================================
  Files        141     141             
  Lines      11342   11335      -7     
=======================================
- Hits        5921    5914      -7     
  Misses      5421    5421             

see 4 files with indirect coverage changes

[macflo8]

I see .model.material in both places now. looks good to me