Translate documentation in Spanish

[ICHydro]

Is your feature request related to a problem or limitation? Please describe.
Use mkdocs internationalisation functionality to provide the documentation in multiple languages, especially Spanish

Describe the solution you'd like
Multi-language documentation solution.

[dalonsoa]

Requires some work - and obviously translating all the documentation - but it should be doable. See squidfunk/mkdocs-material#2346

[dalonsoa]

This is going to be very time consuming, so I would suggest we leave this to the end, after we have sorted the technical issues and bugs, time permitting.

[ICHydro]

I can work on this. I have assigned it to myself.