Update the build from sources guide for Ubuntu 20.04 LTS
Since the Stork build system requires Python 3.10 and Ruby 2.3, the guide to build Stork from sources is no longer applicable on Ubuntu 20.04 LTS.
The guide recommends installing Python and Ruby from the package repository, but the available versions of the above dependencies are outdated. Users need to install Python and Ruby from their maintainers' external, official packages. We should mention it in the guide.