Improve project documentation

[hidmic]

Feature description

Following #176 and other offline comments we've received, the what, why, and how of Beluga are not clear in our current documentation. It needs improvement and clarification.

Implementation considerations

I'm inclined to add Doxygen documentation pages in both beluga and beluga_amcl. We post the former in https://ekumen-os.github.io/beluga, the latter will be posted in docs.ros.org once we release.

[hidmic]

As of #346, I think we can call this done.