Improve minimal ellipses SDP demonstration

[maxkapur]

This PR improves the "Minimum ellipses" demonstration in the documentation:

• Fixed the objective function to use the log determinant of the matrix instead of the trace.
  (Logdet is correct, but the MOI.RootDetConeTriangle interface wasn't working at the time the previous version of this demo was created, so the author used trace as an approximation.)
• Allow input ellipses that aren't centered at the origin.
• Add section number to textbook reference.
• Add graphical output.
• Change title from "Minimum ellipses" to "Minimal ellipses."

See this discussion on Discourse.

[codecov]

Codecov Report

Merging #2945 (bfbd05c) into master (e8d656c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2945   +/-   ##
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  Coverage   95.42%   95.42%           
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  Files          43       43           
  Lines        5794     5794           
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  Hits         5529     5529           
  Misses        265      265           

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[odow]

Thanks for the PR!

Two comments:

• The documentation build is failing https://github.com/jump-dev/JuMP.jl/runs/5967440505?check_suite_focus=true. I've left some in-line comments with what needs to change
• To ensure consistency, we run a code formatter that is failing. Take a look here, https://github.com/jump-dev/JuMP.jl/runs/5967440511?check_suite_focus=true, and implement the changes it suggests.

Once you fix the failures, it'll make a preview of the documentation and we can see how your plot looks 😄

[odow]

Looks much better