Improved documentation website

[raoulcollenteur]

This PR bring a totally new documentation website to Pastas, using a new theme. On the new website is a lot more clear where to find things due to better navigation options for the theme. The API docs have been strongly improved, summarising different modules and classes and providing a better overview of all the methods and classes available in Pastas (see e.g., https://pastas.readthedocs.io/en/improve_docs/api/statistics.html). These are now focused on Users instead of developers.

Basically no information was removed, and only information was added. There are still some sections to fill (help welcome) but most of the work is done. Most importantly, the new website allows us to better structure information and make more information available over time.

[rubencalje]

Looks good! I think this will help people a lot. Two remarks:

• On my phone the menu button does not do anything. Do you know why this is the case?
• Some items are empty, like the about section. Maybe we could remove those for now?

[dbrakenhoff]

See comments in code

[raoulcollenteur]

• On my phone the menu button does not do anything. Do you know why this is the case?

No idea why that happens. Let's hope people don't check on their phone to often..

• Some items are empty, like the about section. Maybe we could remove those for now?

I removed empty items but mostly filled them.