Note about old and new Debian package channels

[cerna]

This pull request adds note about new cloudsmith.io/~machinekit based channels to the documentation in place where user is advised to install old deb.machinekit.io based ones.

This is partially solving issue described in #321 (and is actually putting lipstick on pig - but it will do for now).

[cerna]

It was marinading long enough...

[the-snowwhite]

@cerna It seems like this commit somehow has disappeared (has no check log) and is also not to find in the docs

[the-snowwhite]

The full error is:
Run str="machinekit/machinekit-docs"
Error: Unable to process command '::set-env name=PARSED_USERNAME::machinekit' successfully.
Error: The set-env command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable to true. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
Error: Unable to process command '::set-env name=PARSED_LOCAL_REPOSITORY::machinekit-docs' successfully.
Error: The set-env command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable to true. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/