Import command docs should state why they are useful

[JOJ0]

Import commands docs chapters should state more clearly which parts of DiscoDOS make use of the imported data

Commands reference aka CLI --help:

• Update import basic --help. Done. Latest version here.
• Update import tracks --help --help states short but already what the purpose is, this can be still improved...
• Update import brainz --help

User manual chapters:

• Update import basic manual chapter.
• Update import tracks manual chapter. The manual chapter has been improved and now links to the actual commands that use it: https://discodos.readthedocs.io/en/latest/MANUAL.html#importing-track-details
• Update import brainz manual chapter.https://discodos.readthedocs.io/en/latest/MANUAL.html#importing-further-track-details-from-musicbrainz

[JOJ0]

@Samplissimo some progress. Early feedback will be appreciated! Is import tracks --help and the import tracks manual chapter good enough / understandable enough already?

Also the import basic --help got a lot of more details. Is that understandable? Or too many details?