Rollup merge of #37438 - Cobrand:index_doc, r=GuillaumeGomez
Rollup merge of #37438 - Cobrand:index_doc, r=GuillaumeGomez

improve docs for Index and IndexMut

This mainly changes the boring example of Foo/Bar of `IndexMut` into a better one.

Also added explanations about syntactic sugar for `v[index]`.

Closes #36329

graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust