[mlir][docs] Add page for third-party tutorials (#188080)

Add a new page to the MLIR documentation that links to the
upstream Lighthouse project as well as additional third-party tutorials.
The goal is to make it easier for newcomers to discover MLIR learning
resources beyond the Toy tutorial.

The underlying discussion/RFC can be found
[here](https://discourse.llvm.org/t/rfc-tutorial-a-beginner-friendly-end-to-end-mlir-compiler-pipeline/89788).

GitOrigin-RevId: b87fe380d7cdb9b3662a2c2c4e10b8a1a3ce0404