Added Markdown file partition_point.md in algorithms folder

[carolinekraus02]

I added a description and an embedded code sample in line with the contribution guidelines.

This PR incorporates a new function in the algorithms folder, partition_point. I created a partition_point.md file following the examples from other functions.

Changes

I added a new function in the algorithms folder, partition_point, in the partition_point.md file. I added a description, a code sample, and a link to RexTester showing the code sample.

Checklist

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