Refactor: separate prompts from agent code

Implementations of `BaseDoclingAgent` are found in [agent](https://github.com/docling-project/docling-agent/tree/main/docling_agent/agent) folder.
Those agents leverage hard-coded prompts or template prompts that are spread among public and private methods.
Those prompts are sometimes disconnected and may be inconsistent across agents.
In other cases, the chat prompt may overlap with the system prompt and create inefficiencies.

This issue consists of:
- refactoring the agents by exporting all prompts in a separate module
- align prompt style across agents