Add Nemo/matrix.md to docs/doc.main

[JohnAAbbott]

So that some Nemo matrix operations are visible in the Oscar doc.
Actually I don't even recall making this change... just found it "at the back of a drawer".
Feel free to shoot it down if it is not constructive.

[JohnAAbbott]

Superseded by PR #6078

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.21%. Comparing base (5ee190d) to head (3394dc7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5984   +/-   ##
=======================================
  Coverage   84.21%   84.21%           
=======================================
  Files         787      787           
  Lines      107371   107371           
=======================================
  Hits        90426    90426           
  Misses      16945    16945           

see 3 files with indirect coverage changes

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