Document `--metadata-file` flag alongside `--metadata` in CLI

[mcanouil]

Currently only -M, --metadata is documented in Quarto CLI while --metadata-file is a pretty convenient way to pass complex metadata.

For reference, there is some inconsistent handling depending on how metadata are given:

• metadata override through CLI flags is inconsitent #6123

[cderv]

I wonder if we document it now despite knowing limitations of

• metadata override through CLI flags is inconsitent #6123

I believe --metadata-file was adding later on, and it does not handle great Quarto only metadata;

It currently works well as a usual Pandoc CLI flag. So for now, it is really "documented" as

pandoc-args...                      - Additional pandoc command line arguments. 

😅