Add HTML to Markdown functionality

### Problem Description

Markdown is the de facto standard for LLMs, so having an HTML -> markdown conversion functionality would help users who want to build RAG applications.

### Proposed Solution

Add an optional HTML -> Markdown functionality

- [ ] Configurable in the YAML file
- [ ] When configured, adds a single field to ingested document, containing the converted markdown content
- [ ] When not configured, field is not present on ingested documents at all