Add support for markdown documents

[pmeier]

Markdown is the base format for tools like Obsidian or logseq. As a user, I would like to be able to ask question about all the notes I took.

Since we already support parsing plain .txt files with

ragna/ragna/core/_document.py

Lines 226 to 231 in 4962665

	class TxtDocumentHandler(DocumentHandler):
	"""Document handler for `.txt` documents."""
	@classmethod
	def supported_suffixes(cls) -> list[str]:
	return [".txt"]

we could just rename it to PlainTextDocumentHandler and add .md to the supported suffixes.

[paskett]

I'd like to help contribute to ragna. This seems like some low-hanging fruit as a first issue and to set up my development environment.