add `required` flag to attribute includes content.md

[jmvera255]

currently only the tables in the template pages show information about whether an attribute is required. It would be useful to also render this info on the individual attribute pages. this can be done by adding a line to the attribute's corresponding _includes content.md file.

[jmvera255]

this will have to be reconsidered or reimagines b/c of the effects of conditionals such that attributes that are marked as required are actually only required in certain contexts and/or valid value selections for certain attributes so it's not a simple "yes"/"no" to display.