Update set rend to match style guide

[sometallgit]

This PR makes changes to rend.md to conform to the contributing.md style guide. It also adds a corresponding rend.cpp code snippet.

Changes

• Remove <bits/stdc++.h> header and replace it with the correct <set> include.
• Update set to use list initialization rather than copying from a C-style array.
• Remove using namespace std.
• Update run code example.
• Add code snippet.

Checklist

• [X] I have read CONTRIBUTING guidelines.
• [ ] This is a typo fix.
• [X] I am not updating any todo.txt files.