[docs] Simplify the introduction page

[odow]

Closes #2394

[codecov]

Codecov Report

Merging #2395 (c6fb391) into master (f47b18c) will not change coverage.
The diff coverage is n/a.

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  Coverage   91.61%   91.61%           
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  Files          42       42           
  Lines        4451     4451           
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  Hits         4078     4078           
  Misses        373      373           

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[mlubin]

Perhaps a TODO: I think it would be useful to have a page that explains the concept of an algebraic modeling language and what JuMP does do and doesn't do. The documentation is missing this kind of summary that would help people decide if they want to use JuMP or not. Jumping straight to the installation guide is making an assumption that somewhat has already decided to install JuMP, but maybe they're browsing the documentation to decide if they want to install JuMP.

[odow]

Opened an issue #2412