Release v2.0.0

This release introduces the robotology-superbuild infrastructure to build the Docker images as specified in #30.
Thus, we can tag the Docker images with proper versions.

In particular, this release is shipped with the tagged image v2021.05, yielded based on 📦 Distro 2021.05.

Moreover, we need now to do sudo make install of the project as the superbuild installed products cannot be put under /workspace (it is overwritten at the Gitpod workspace creation time), which is the only non-ephemeral location so far. The latter will be fixed by the upcoming Gitpod feature call "Full workspace backup".