More docs & a few fixes (#583)

Docs for all non-obsolete methods in AngouriMath.MathS added.

Bug fixed: there was a wrong definition of hyperbolic cotangent (MathS.Hyperbolic.Cotanh).

Added API for: `MathS.oo`. Use:
```
using static AngouriMath.MathS;

var inf = +oo;
var minf = -oo;
```

Also added API for creating multiple variables:
```
using static AngouriMath.MathS;

var x = Var("x"); // old API
var (x, y) = Var("x", "y"); // new one
var (x, y, z) = Var("x", "y", "z"); // new one
```

Docs tracking issue: #585 .