Rollup merge of #108094 - kornelski:fsdocs, r=cuviper

Demonstrate I/O in File examples

I've noticed that some Rust novices unnecessarily reinvent `std::fs::{read,write}`, presumably because they search for equivalents of `fopen` + `fwrite`. I've added links to `std::fs::{read,write}` in the docs.

The `File` examples were only showing how to open a file, but not how to use the opened handle, unnecessarily leaving out the next step. I've added a variety of different uses of file handles to their examples in docs.

Author: matthiaskrgr