Add allowed-value constraints tutorial

[aj-stein-nist]

Committer Notes

Begin the allowed-value constraints tutorial and potentially others, pending feedback. Closes #292.

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[aj-stein-nist]

I would like feedback before removing the draft status and final review.

1. Specific, actionable grammar and spelling changes are always welcome. 😛
2. After expanding our required and recommended value strategies, do we encourage me or require me to add a third section on "extending" constraint definitions and handling conflicts? That seems like an advanced topic, but relevant to the constraints tutorial here.
3. Should I continue beyond allowed-values constraints tutorials and come up with examples of <index/>, <is-unique/>, <expect/>, and <matches/> or should that be separately scoped work for a separate tutorial or, as is convenient for me, include it here? (this will be Tutorial 2, I am implying with this answer you recommend the subject matter be Tutorial 3, or even 5 or 6 in a different order of complexity.)

[aj-stein-nist]

I also did some touch-ups on this to relabel the introduction correctly, highlight changes to the model, and more interestingly highlight relevant changes to the example document instances. I didn't realize the tabs and highlight wrapped directives were native Hugo configuration with the templates we use working out of the box, so I added that as well and it just works.

[david-waltermire]

This looks great! Thank you so much for all the hard work on this.

I provided some non-blocking feedback, which could be addressed in another iteration. I believe this tutorial could be released as-is.

[aj-stein-nist]

Sorry for the delays, I will try to touch this up before the community call today.

[aj-stein-nist]

Re #508 (comment), I only got feedback from @david-waltermire that I should separate out the constraint type tutorials and not keep them in one massive tutorial. As this is tutorial 2, follow-on constraint tutorials will be 3, 4, or maybe 5 respectively. Will ask in today's meeting for more feedback but likely move to merge this once I apply PR recommendations and mark for final review and tentative merge. I will open new issues for the follow-on tutorials.

[aj-stein-nist]

Sorry for the delay, this is ready for final review with feedback from Updated in 889758b and feedback integrated.