Update docs with `uvx` instructions for fast CLI workflow

[muuvmuuv]

Just came across this awesome project. I had to convert a lot of MediaWiki pages and doing MediaWiki → PDF → Markdown was the easiest choice for 1000+ pages. So, thanks, awesome work!

Now, due to the reason, that I am not having pip installed globally, I have used uvx from Astral. I think it would be great to add that to the docs additionally to pip install when users just wanna process a file.

For example:

uvx --from marker-pdf marker_single --output_dir $(pwd) MyAwesomeArticle.pdf