Add link to functions and constants documentation

[ChristophB]

Background

Documentation for functions and constants is currently provided in the localisation files. We have to review and modify these files whenever top-phenotypic-query is updated.

Requested Changes

• [x] introduce environment var with URL to top-phenotypic-query documentation, default: https://Onto-Med.github.io/top-phenotypic-query
• [x] provide some kind of deep link to the documentation from a specific function/constant (<base_url>/<function_type>/<function_id>)
• [x] drop function/constant descriptions from src/i18n/*.json

[ChristophB]

This solution does not work for text functions as they are included in another repository/package and they have no documentation yet.