Add an example for Vec::splice inserting elements without removing

[cod10129]

This example clearly showcases how splice can be used to insert multiple elements efficiently at an index into a vector.

Fixes #135369.

The added example:

Using splice to insert new items into a vector efficiently at a specific position indicated by an empty range:

let mut v = vec![1, 5];
let new = [2, 3, 4];
v.splice(1..1, new);
assert_eq!(v, [1, 2, 3, 4, 5]);

@rustbot label A-docs A-collections

[rustbot]

r? @Mark-Simulacrum

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[tgross35]

Thanks for the change, lgtm.

@bors r+ rollup

[bors]

📌 Commit b11f87a has been approved by tgross35

It is now in the queue for this repository.