Add conditional assignments to the language tutorial

[susan-garry]

The language tutorial for calyx, found here, does not include any information about the ? and & symbols. These are key features of the language, but I haven't been able to find any information about it in the documentation (they are used in examples under debugging tips, but their semantics are not explained). Information about these keywords should probably be added to the tutorial.

[rachitnigam]

Ah, good point! We should probably explain this somewhere in detail.

[sampsyo]

Yes indeed! I think it would be cool to add this in another stage in the language tutorial… it could be kinda contrived, but we could just add a phase where the loop only adds up odd numbers, skipping the even ones, or something like that.