feat: add es2n

[MartinuzziFrancesco]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This PR adds a new cell ES2NCell and a new model ES2N. I think I could also use the ES2NCell implementation as a documentation example, where I illustrate how it is possible to simply build cells, and feed them to ReservoirComputer to get some exploration starting.

• Implement ES2NCell
• Document ES2NCell (docstrings)
• Test ES2NCell
• Implement ES2N
• Document ES2N (docstrings)
• Test ES2N
• Add building of ES2NCell as a documentation example. ES2N is then built using ReservoirComputer.

closes #325, address part of #131 by getting started with adding some examples