"Building from sources" guide is outdated

[rohrerj]

The content of the site https://docs.scionlab.org/content/install/src.html is outdated.

1. Ubuntu 16.04 might not be recent enough anymore. SCION docs says "Ubuntu release 18.04 or later".
2. Configuration Step 4 mentions to run supervisor/supervisor.sh reload but this folder no longer exists. The script has been moved to the tools folder.
3. The same is true for the Running SCION section.