DOC: Document command-line options

[larsoner]

When looking at https://github.com/mne-tools/mne-bids-pipeline/pull/914 it occurred to me that command-line options like --no-cache are not documented anywhere as far as I can tell. This could perhaps be added as another section in the configuration docs, perhaps with cross-links from config options like memory_method or whatever

[hoechenberger]

I like the idea