Update docs index to use lists

Also include a missing link to the rustdoc manual
auto merge of #11588 : am0d/rust/dox, r=brson

This is a bit cleaner and means we don't need whitespace at the end of each line to force line breaks.
Add a generic power function

The patch adds a `pow` function for types implementing `One`, `Mul` and
`Clone` trait.

The patch also renames f32 and f64 pow into powf in order to still have
a way to easily have float powers. It uses llvms intrinsics.

The pow implementation for all num types uses the exponentiation by
square.

Fixes bug #11499