Add no-alt-text rule to accessibility.json

[khiga8]

I am understanding the accessibility.json rule to have configurations explicitly for accessibility-focused rules. (let me know if my understanding is wrong)

This rule will already be pulled in by the default: true setting in base.json, but I think we should explicitly enable it here too.

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[khiga8]

@andrialexandrou I'd appreciate if you could provide background on the setup of base.json and accessibility.json! 🙇‍♀️

[andrialexandrou]

@khiga8 correct, the idea is to identify which rules are specifically related to accessibility.

If we want to, in the future, configure tracking around discouraging teams disabling accessibility-related markdown rules, then we've identified which ones we care about.