[tutorials] Clarify that glTF is our preferred mesh format

[jwnimmer-tri]

(We will squash this for merge.)

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This change is 

[jwnimmer-tri]

I came across this stale doc when doing other work. This was a good stab at fixing it, and I think I found all of the tutorial bugs re: obj files, but someone should cross-check me.

So let's do +@SeanCurtis-TRI for feature review please, and maybe just push fixes for anything I missed?

[jwnimmer-tri]

On second look, maybe we should add a hyperlink to https://drake.mit.edu/doxygen_cxx/group__geometry__file__formats.html from somewhere in here?

[jwnimmer-tri]

on the edits.

+@ggould-tri for platform review per schedule, please.