Document use of `den` for --output-spaces

[emdupre]

I was looking at the documentation on using the (new to me) --output-spaces argument, and I can't seem to find any included examples with the den parameter (i.e., for surface space). I see from #1588 that the call should look something like:

--output-spaces fsaverage:den-10k

Is this in fact supported in the most recent fMRIPrep ? If so, should we add this to the documentation ? I'd be happy to open a PR !

[effigies]

I think we support this because that keyword exists in https://github.com/templateflow/tpl-fsaverage/tree/328ebe36e43d208d5d31628f92eb21a238aedebc.

I agree this needs to be better documented, and if possible, give people the tools to find the information without having a brain full of our web of semi-related repositories.

[oesteban]

Surface templates are not completely supported, but yes, the spirit is that den- works well and it indeed needs to be documented.

[utooley]

I'm happy to tackle this, I think I understand FS resolutions! Although from #1843 it looks like it isn't fully working yet...

Is this appropriate to tack on minimal guidance about this to the paragraph about specifying surface spaces in the spaces section? It looks like there might be eventually be more guidance we could link to here about res- and den-, but in the meantime.