Document supported file types for /learn

[JasonWeill]

Fixes #406. Documents a list of supported file types for the /learn command, as mentioned here:

jupyter-ai/packages/jupyter-ai/jupyter_ai/document_loaders/directory.py

Lines 85 to 101 in 465d991

	SUPPORTED_EXTS = {
	".py",
	".md",
	".R",
	".Rmd",
	".jl",
	".sh",
	".ipynb",
	".js",
	".ts",
	".jsx",
	".tsx",
	".txt",
	".html",
	".pdf",
	".tex", # added for raw latex files from arxiv
	}

The relevant section of the docs: https://jupyter-ai--816.org.readthedocs.build/en/816/users/index.html#supported-files-for-the-learn-command

[dlqqq]

Thanks!