umath: Redo module docs.

This reworks the documentation for the umath module, as done previously as per https://github.com/pybricks/pybricks-api/pull/64 and https://github.com/pybricks/pybricks-api/issues/63

The presentation and docstrings are inspired by the Python 3 math module documentation instead of the MicroPython math module documentation. Functions are grouped roughly by use case.

doc/api/umath.rst

@@ -1,4 +1,77 @@
-:mod:`umath` -- Mathematical functions
-======================================
+:mod:`umath <umath>` -- Math functions
+============================================================
 
-.. automodule:: umath
+This module is available on the the City Hub, Technic Hub,
+Prime Hub, and Inventor Hub.
+
+This MicroPython module is similar to the `math module`_ in Python.
+
+.. module:: umath
+
+Rounding and sign
+-------------------------------------
+
+.. autofunction:: umath.ceil
+
+.. autofunction:: umath.floor
+
+.. autofunction:: umath.trunc
+
+.. autofunction:: umath.fmod
+
+.. autofunction:: umath.fabs
+
+.. autofunction:: umath.copysign
+
+Powers and logarithms
+-------------------------------
+
+.. autodata:: umath.e
+
+.. autofunction:: umath.exp
+
+.. autofunction:: umath.pow
+
+.. autofunction:: umath.log
+
+.. autofunction:: umath.sqrt
+
+Trigonomety
+-------------------------------
+
+.. autodata:: umath.pi
+
+.. autofunction:: umath.degrees
+
+.. autofunction:: umath.radians
+
+.. autofunction:: umath.sin
+
+.. autofunction:: umath.asin
+
+.. autofunction:: umath.cos
+
+.. autofunction:: umath.acos
+
+.. autofunction:: umath.tan
+
+.. autofunction:: umath.atan
+
+.. autofunction:: umath.atan2
+
+Other math functions
+-------------------------------
+
+.. autofunction:: umath.isfinite
+
+.. autofunction:: umath.isinfinite
+
+.. autofunction:: umath.isnan
+
+.. autofunction:: umath.modf
+
+.. autofunction:: umath.frexp
+
+.. autofunction:: umath.ldexp
+
+.. _math module: https://docs.python.org/3.5/library/math.html#module-math