docs: use local Debian packages for bare-metal deployment

See merge request !6

# Conflicts:
#   docs/bare-metal-installation.md

Grammar and other documentation issues

See merge request !7

# Conflicts:
#   src/engine.py
#   src/mixer.py

fix(heartbeat-monitor): port the "PyHeartBeat" script to Python3

See merge request !8

docs: fix spelling issues

See merge request !4

Improve graceful shutdown. #88

See merge request !10

Thanks, codespell!

The Debian packages benefit from security support and should be much
easier to install for most users.

Currently all requirements of Auro Engine are packaged in Debian.
Thus the virtualenv-related documentation snippets can be removed.

In addition to porting it to python3, a number of documentation and code
issues were solved:

* doc: proper order of arguments
* doc: mention debug mode (previously via "__debug__", now "DEBUG=1")
* doc: provide example for manual heartbeat packets
* code: specify timeout for "socket.recvfrom" (avoid hanging)
* code: handle parameters properly (cast to int/float)
* style: format with "black"