Update link to Fabrik documentation on homepage

- Currently, the link to the Fabrik documentation in the "PROJECTS" section of the [homepage](https://cloudcv.org/) points to the outdated [Fabrik Wiki](https://github.com/Cloud-CV/Fabrik/wiki) (see this Fabrik issue: https://github.com/Cloud-CV/Fabrik/issues/522).
- For consistency, because the documentations of both EvalAI and Origami point to their `readthedocs.io` pages, it would be better to change this link to https://fabrik.readthedocs.io/en/latest/ .