Respect XDG base dir spec for config file

[baodrate]

Basic fix to use the XDG_CONFIG_HOME env var if it exists. Doesn't do more sophisticated platform-specific handling (it's an option but I'd personally hand that off to something like platformdirs).

closes #302

[baodrate]

Logfile is currently set to {CONFIG_HOME}/error.log. It would be better to have this be {XDG_CACHE_HOME}/bpytop/error.log or something similar, but I'm not sure if it's worth changing.

[baodrate]

Tried to update the documentation and default config file to indicate the new config directory. Examples use shell expansion to indicate behavior, suggestions for alternative formatting are welcome.