[docs] add example to index.md

[odow]

Our landing page has a lot of text with no example: https://jump.dev/JuMP.jl/v1.23.1/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.90%. Comparing base (9fb707c) to head (95d6078).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3820   +/-   ##
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  Coverage   97.90%   97.90%           
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  Files          44       44           
  Lines        6022     6022           
=======================================
  Hits         5896     5896           
  Misses        126      126           

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[odow]

Now it is

I think it's an improvement to have an example of using JuMP on the first page of the docs.