Improve Documentation for Carrier Creation

[MarcoWel]

Is your feature request related to a problem? Please describe.
The carrier creation is not documented well at the moment and hard to get right.

Describe the solution you'd like
The fields to enter should be described better, and there should be a clear documentation for each carrier.

Additional context
Example: Deutsche Post DHL

• User has to create an app here and get it approved by DHL first: https://entwickler.dhl.de/group/ep/myapps
• The production endpoint and credentials will be used.

EDIT: Currently, the account number needs to be EKP with added postfix (e.g. 0101), see karrioapi/karrio#333

[MarcoWel]

DHL Universal:

• API Docs: https://developer.dhl.com/tracking#get-started-section/
• Prerequisites: Register and create app here: https://developer.dhl.com/user/apps

[MarcoWel]

UPS:

• Prerequisites: Request API token here: https://www.ups.com/upsdeveloperkit

[MarcoWel]

DHL Express:

• Prerequisites: Register here: https://developer.dhl.com/form/dhl-express-onboarding

[MarcoWel]

Deutsche Post DHL (Test Mode / Sandbox):