Minor improvements in documentation

[jmejia8]

I am working on my review for JOSS: openjournals/joss-reviews#4736

The package seems well-coded and structured. However, some minor improvements can be made in the documentation:

• Maybe this table of contents should be inserted in a different section

  PDENLPModels.jl/docs/src/index.md

  Line 26 in 389eed1

  ```@contents

• It might be helpful to provide some bibliographical references for these examples:

PDENLPModels.jl/docs/src/poisson-boltzman.md

Line 2 in 389eed1

## Poisson-Boltzman problem

PDENLPModels.jl/docs/src/tore.md

Line 2 in 389eed1

## The Brachistochrone over a Tore

• Please, check the syntax highlighting in the examples mentioned above.

[jmejia8]

Solved in #84
@tmigot, thank you!