Use HTML relative paths for tiles in the docs
This PR replaces the absolute paths in the landing page tiles with relative ones so that one can test navigation both locally in and in future PRs (see [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5084/en/index) for an example PR where the links don't work).

I encountered this while working on the `optimum` docs and figured I'd fix it elsewhere too :)

Internal Slack thread: https://huggingface.slack.com/archives/C02GLJ5S0E9/p1665129710176619