Add documentation about unit conversion feature

[naveen521kk]

Enhancement proposal

This feature was implemented in https://github.com/ManimCommunity/manim/pull/742, but there isn't any documentation about this.

# 50px -> Munit
>>> 50 * unit.Pixels
0.37037037037037035
# 90 degrees to radians
>>> 90 * unit.Degrees
1.5707963267948966
# 10 % of X-axis
>>> unit.Percent(X_AXIS) * 10
1.4222222222222223

[tameTNT]

Hi! I would be prepared to write some documentation and examples for this if that's okay?

[ad-chaos]

Go ahead!