add section about projects that uses XLS

It would be nice to have a section in the documentation https://google.github.io/xls/ that links to related projects that extends or uses XLS

We could link to:
- https://github.com/google/xls/tree/main/xls/contrib
- https://github.com/google/xls/tree/main/third_party
- https://github.com/xlsynth/xlsynth-crate
- https://github.com/antmicro/xls-dashboard
- https://github.com/antmicro/xls-vizier

We could have something in the style of awesome pages like https://github.com/drom/awesome-hdl or https://github.com/mattvenn/awesome-opensource-asic-resources