Proposal: Additional (Trip) Notices

### Describe the problem

Many data standards like NeTEx and HRDF offer data structures to provide additional human-readable notes assigned to trips in order to inform about specials not covered in technical properties or which shall be communicated in a certain, explicit wording to the passener. GTFS does this actually not, some data providers use informational service alerts for that (e.g. travic.app states additional notes under 'service alerts'). However, service alerts belong to the realtime data and are (for my understanding) meant to inform about unplanned changes. The additional notes I mean are already known during conceptual planning and belong not really to the scope of realtime data.

### Use cases

Adding any arbitrary, non-technical, but already known notice to a trip, e.g.:
- "Bike reservation recommended"
- "Special Train - Dedicated fare is required"
- "Express Bus - Please note the special trip times"
- "Sesonal Trip - Runs during the XYZ festival"

### Proposed solution

First draft:
- Add a file notices.txt (notice_id, display_text, [tts_text])
- Add a file trip_notices.txt (trip_id, notice_id)

Before I investigate more time here, I'd like to hear some of your opinions regarding this topic. I believe, I am not the only one having this issue.

### Additional information

We must be careful when imolementing an extension like that, because it might lead to 'misuse' of the notices instead of filling already existing technical fields properly.