docs: persistent configuration in Docker

[m0wer]

So that the configuration is persisted on the host when running ctop with Docker.

[Guiorgy]

Great addition. The only thing I'd change is to use a managed volume instead of a bind, so the default works on any system without modification. If the user wants to use a bind instead, they are free to modify the command themselves:

-e XDG_CONFIG_HOME=/conf \
-v ctop_conf:/conf/ctop/ \