Allow for simpler, less error-prone example front matter

[abey79]

Currently, example's front matter include full path to the main file, potentially linking to a rust version as well, e.g.:

---
title: Objectron
python: https://github.com/rerun-io/rerun/tree/latest/examples/python/objectron/main.py
rust: https://github.com/rerun-io/rerun/tree/latest/examples/rust/objectron/src/main.rs
tags: [2D, 3D, object-detection, pinhole-camera]
---

This is error prone on many level: absolute path when relative would be good, rust stuff referenced from python examples, etc.

Ideally, the front matter would only include a single source: main.py with a relative path. If a given example exists in both python and rust, the CI should deal with it as needed:

• to match the current behaviour, pull the main: path from the rust example and add its link into the web page (which is currently mainly based on the python's readme)
• probably in the future, both versions of the example should exist in the landing page, under respective python/rust categories/filters (ideally, the system should be smart enough to notice that alternate implementations exist for a given implementation and offer cross links)