Improve (L-)BFGS docs

[kellertuer]

This is an approach to #1163

Since even after very many tries I did not get beyond a few cross reference errors on the examples – they do render but something seems still wrong, I will try this PR to maybe render and look at the docs.

[github-actions]

Benchmark Results

	master	2d16fd7...	master / 2d16fd7...
multivariate/solvers/first_order/AdaMax	0.643 ± 0.0078 ms	0.645 ± 0.0077 ms	0.997
multivariate/solvers/first_order/Adam	0.643 ± 0.0076 ms	0.646 ± 0.0078 ms	0.995
multivariate/solvers/first_order/BFGS	0.224 ± 0.0053 ms	0.223 ± 0.006 ms	1.01
multivariate/solvers/first_order/ConjugateGradient	0.048 ± 0.0006 ms	0.0478 ± 0.00062 ms	1
multivariate/solvers/first_order/GradientDescent	1.71 ± 0.014 ms	1.7 ± 0.016 ms	1
multivariate/solvers/first_order/LBFGS	0.219 ± 0.0041 ms	0.218 ± 0.0038 ms	1.01
multivariate/solvers/first_order/MomentumGradientDescent	2.5 ± 0.019 ms	2.5 ± 0.021 ms	0.999
multivariate/solvers/first_order/NGMRES	0.556 ± 0.011 ms	0.553 ± 0.011 ms	1.01
time_to_load	0.535 ± 0.0098 s	0.534 ± 0.0073 s	1

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[kellertuer]

Got it to work locally and also fixed the literature section to print the literature correctly. One could also replace that in the doc strings (wherever they are rendered).

So this is the form I would propose for the L-BFGS docs then.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.70%. Comparing base (8a55bc3) to head (2d16fd7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1164      +/-   ##
==========================================
- Coverage   85.75%   85.70%   -0.05%     
==========================================
  Files          46       46              
  Lines        3593     3596       +3     
==========================================
+ Hits         3081     3082       +1     
- Misses        512      514       +2     

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[kellertuer]

I think I am finished with this and if it is ok it could be merged, @pkofod – if your time allows.