Update platform maintenance status

Description

The installation page has a number of supported platforms, but some of then are unmaintained right now.

Tasks

• Add a general note on platform support (and ask for help).
• Add a note on unmaintained platforms (and ask for help):
  • Raspbian 9.
  • FreeBSD.
  • MacOS.
  • CentOS.

Time estimation

• Complexity: negligible (0.1 day)
• Uncertainty: low (x1.1)
• Reference (adapted)