Work on the documentation

[fedebenelli]

There is little to none documentation since the API rework. So it should be updated.

Also user level pages should be added to ease up the understanding process for the end-user.

[fedebenelli]

API documentation acceptable. Still need to add individual pages for user documentation

[fedebenelli]

• [ ] Individual pages
• [ ] Check correct documentation of properties (for example, fugacity_vt gives $\phi_i P$ and not $f_i$)