Improve Beluga landing page

[hidmic]

Feature description

Beluga's documentation landing page needs some work. A potential user should walk away with a clear understanding of why Beluga exists, what it offers, and why should they use it over the alternatives. Nav2's landing page is a good reference for this (https://navigation.ros.org).

Implementation considerations

Whichever tooling we decide to use for high-level documentation, it should be the same as that used for low-level documentation so that the whole user experience is consistent.