use concat_url to add "/parse" to duckling url safely

[erohmensing]

Proposed changes:

• use concat_url instead of basic string concatenation to handle trailing slashes on the URL
• fix Duckling url with trailing slash leads to parsing failure  #5792

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[tabergma]

Looks great! 🚀