Fixes links for mesa examples in ReadMe.

[Sahil-Chhoker]

Summary

Fixed link references for examples in Readme

Bug / Issue

Closes #2527

[EwoutH]

This was a conscious tradeoff. While this changes fixes the links in the GitHub Readme, it breaks our links in the ReadtheDocs. See #2448.

To make this work, I had to slightly change the links in the example headers. They don't work anymore on GitHub, but do work on RTD. I think this is a worthwhile tradeoff. If you read the readme on github, you have the file explorere at the top of the page anyway. I tested six or so versions of the links but none worked for both.

Closing this PR, if you find a solution that works for both feel free to open a new PR.