Clarify documentation of choose_weighted(_mut) mentioning accurate behavior with floats

[ISibboI]

Changes as discussed in this comment in #1243

• Updates the choose_weighted (and similar) methods to have links to the WeightedIndex distribution.
• Removes any links to deprecated modules
• Explicitly mentions that zero weights (for floats and ints) are handled correctly, perhaps right before the example using weights of zero in the WeightedIndex docs.

I added the mention of zero weight in the first paragraph of the documentation.

Additionally

• I fixed two minor issues in the docs of WeightedIndex and
• changed the example that includes a weight of zero to use floating point numbers.

Feel free to request changes, this is my first contribution to this repository, so I don't really see the big picture.

Closes #1243

[dhardy]

Thanks, this looks very good. (Sorry for the delay!)

[ISibboI]

No worries, thanks!